Direct memory access controller having programmable timing

ABSTRACT

An improved DMA controller having programmable data transfer timings. Not only is the total cycle time programmable, but the active and inactive period of the cycle are also programmable. An active timing register and an inactive timing register are used in conjunction with a countdown timer to determine the active and inactive periods of the data transfer cycle. The active time period is loaded into the timer during the active phase, with the end of the active phase being indicated by the timer timing out. Next, the inactive time period is loaded into the timer, which similarly times out to indicate the end of the inactive phase of the data transfer cycle.

This is a continuation of U.S. patent application Ser. No. 08/699,548filed Aug. 19, 1996, now U.S. Pat. No. 5,692,216 which is a continuationof U.S. patent application Ser. No. 08/398,358 filed Mar. 3, 1995, nowU.S. Pat. No. 5,603,050.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to direct memory access (DMA) controllers, andmore particularly, to an improved DMA controller having programmabletiming.

2. Description of the Related Art

To improve overall computer system performance, computer systems todayinclude a direct memory access or DMA controller to enable I/O devicesto access main memory without the intervention of the CPU. In computersystems according to the Industry Standard Architecture (ISA) orExtended Industry Standard Architecture (EISA), two 8237 DMA controllersfrom Intel Corporation are utilized, or in the alternative, an8237-compatible DMA controller can be used.

The DMA controller in the ISA computer system supports either 8 or16-bit I/O devices, while the DMA controller in the EISA system furthersupports 32-bit I/O devices. The DMA controller in an ISA or EISA systemprovides seven DMA channels, each channel capable of being programmedfor several different transfer modes and timing modes. A DMA channel canoperate in one of four transfer modes, which include the singletransfer, block transfer, demand transfer, or cascade modes. In singletransfer mode, a DMA channel performs only one data transfer for eacharbitration cycle. In block transfer mode, a predetermined block of datais transferred in each arbitration cycle. In demand transfer mode, theDMA controller has the capability of continuing the data transfer untilthe I/O device has exhausted its data capacity. Finally, each channel ofthe DMA controller can also be programmed to cascade mode.

One application of cascade mode is where two 8237 devices are cascadedtogether. As each 8237 DMA controller includes only four DMA channels,cascading is required to obtain the seven channels. Thus, one of thechannels of the first level 8237 DMA controller is programmed in cascademode and connected to a second level 8237 DMA controller. This allowsthe DMA requests of the second level device to propagate through thepriority network circuitry of the first level device. Thus, the cascadechannel of the first level DMA controller is used only for prioritizingthe second level device, and it does not output any address or controlsignals of its own as that would conflict with the output signals of theactive channel in the second level device.

Another application of programming a channel to cascade mode is to allowan I/O device on the expansion bus to use a DMA channel for busrequests. Thus, instead of requiring the use of a separate arbiter toarbitrate between the I/O devices, the prioritization scheme between theDMA channels can be used to process requests from the I/O devices. Whenthus programmed, the DMA channel does not perform data transfers to amemory slave. Instead, the output signals of the DMA channel aredisabled to allow the I/O device to gain control of the bus. For moredetailed information on the various transfer modes of the 8237 DMAcontroller, refer to Peripheral Components, Intel Corp., pgs. 5-4 to5-21 (1994), which is hereby incorporated by reference.

The 8237 type DMA controllers also use one of four timing modes: ISAcompatible cycles, Type A cycles, Type B cycles and Type C or burst DMAcycles. It is noted that all the timing modes can be used when the DMAcontroller is programmed in the single transfer mode. However, if the8237 DMA controller is programmed in either the block transfer mode orthe demand transfer mode, then the ISA compatible timing mode cannot beused as there exists the possibility that other devices, including therefresh controller, can be locked out.

In ISA compatible timing mode, one DMA transfer is performed in eightBCLK or ISA bus clocks. In the Type A timing mode, a DMA transfer can becompleted in six BCLK periods. Type B timing provides better performancethan Type A timing, as it supports DMA transfers that can be executedevery four BCLK periods. Both Type A and Type B timings require the useof fast memory devices. When programmed in burst DMA or Type C timingmode, the DMA cycles have characteristics that are similar to burstcycles. Thus, transfers can be performed between the memory slave deviceand the DMA device in one BCLK period each.

As can be seen from the discussion above, the 8237-type DMA controllerprovides for a number of different timing modes to compensate fordifferent timing requirements of different components in a computersystem. Thus, if a computer system is implemented with a fast memorydevice, then the faster DMA timing modes can be utilized. However, thereare also a multitude of I/O devices each having different functional andtiming requirements. It has been determined that the four timing modesprovided by the 8237 DMA controller do not provide sufficientflexibility to optimize DMA performance for the timing requirements ofthe different I/O devices. Therefore, it is desired that a more flexibleand improved DMA controller be provided.

SUMMARY OF THE PRESENT INVENTION

An improved direct memory access (DMA) controller according to thepresent invention allows for complete programmability of the timing of aDMA transfer. Not only is the improved DMA controller capable of varyingthe cycle time required for a DMA transfer, the active and inactiveperiod of each cycle is also programmable. Two timing registers areutilized, one to define the active period and the other to define theinactive period. Active and inactive timing registers are provided foreach of the plurality of channels provided by the improved DMAcontroller. During the active period, the value in the active timingregister is loaded into a count-down timer, which provides a timeoutsignal when the predefined active period elapses. When the active periodelapses, the improved DMA controller transitions to the inactive portionof the DMA cycle, at which time the value in the inactive timingregister is loaded into the timer. When the timer times out, thecompletion of the DMA cycle is indicated. Both the active and inactivetiming registers can be loaded with a plurality of time values, whichprovides for improved timing flexibility over prior DMA controllers.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 is a block diagram of an exemplary computer system includingarbitration logic according to the present invention;

FIG. 2 is a block diagrams of a PCI-ISA bridge in the computer system ofFIG. 1 and incorporating arbitration logic according to the presentinvention;

FIG. 3 is a block diagram of a PCI arbiter in the arbitration logic ofFIG. 1;

FIG. 4 is a block diagram of the reservation and masking logic of thePCI arbiter of FIG. 3;

FIGS. 5A and 5B are a schematic diagram of logic associated with thereservation and masking logic of FIG. 4;

FIG. 6 is a state machine for tracking cycles to indicate when a retryhas occurred to prevent re-prioritization;

FIG. 7 is a state machine for determining when a new master has beengranted the bus;

FIG. 8 is a schematic diagram of logic associated with the state machineof FIG. 7;

FIG. 9 is a schematic diagram of logic and circuitry associated with aminimum grant timer associated with the PCI arbiter of FIG. 3;

FIG. 10 is a block diagram of the LRU arbiter in the PCI arbiter of FIG.3;

FIGS. 11, 12 and 13 are schematic diagrams of circuitry associated withthe arbiter of FIG. 10.

FIG. 14 is a state diagram of a first retry state machine in the PCIarbiter of FIG. 3;

FIG. 15 is a state diagram of an SD arbiter for arbitrating for the dataportion of the ISA expansion bus in the computer system of FIG. 1;

FIGS. 16A and 16B are a schematic diagram of logic in a DMA arbiter tocontrol access of the DMA controller and ISA I/O devices to the ISA bus;

FIG. 17 is a schematic diagram of flush request logic in the computersystem of FIG. 1; and

FIG. 18 is a state diagram of a state machine for monitoring requestsfrom bus masters on the ISA bus of FIG. 1.

FIG. 19 is a schematic diagram of write data buffers in the PCI-ISAbridge of FIG. 2;

FIG. 20 is a schematic diagram of a read data buffer in the PCI-ISAbridge of FIG. 2;

FIG. 21 is a schematic diagram of active and inactive timing registersdefining the DMA transfer timing;

FIG. 22 is a state diagram of an EDMA state machine for controllingwrite and read DMA transfers;

FIGS. 23A and 23B are a schematic diagram of logic for interfacing withthe EDMA state machine of FIG. 22;

FIG. 24 is a state diagram of an IDE state machine for monitoring if theimproved DMA controller of FIG. 2 is in an idle, acknowledge, active orinactive state; and

FIG. 25 is a state diagram of a state machine that controls the writelatching enable signals of the write data buffers of FIG. 19.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, an exemplary computer system S incorporatingthe preferred embodiment of the present invention is shown. In thepreferred embodiment, the system board contains circuitry and slots forreceiving interchangeable circuit boards. In the preferred embodiment,there are two primary buses located in the system S. The first bus isthe PCI or Peripheral Component Interconnect bus P which includes anaddress/data portion and control signal portion. The second primary busin the system S is the ISA bus I. The ISA bus I includes an addressportion, a data portion and a control signal portion. The PCI and ISAbuses P and I form the backbones of the system S.

A CPU/memory subsystem 101 is connected to the PCI bus P. The processoror CPU 100 is preferably the Pentium processor from Intel, preferablyoperating at an external frequency of 66 MHz, but could be an 80486 fromIntel or processors compatible with the 80486 or Pentium or otherprocessors if desired. The processor 100 provides data, address andcontrol portions 102, 104, 106 to form a host bus HB. A level 2 (L2) orexternal cache memory system 108 is connected to the host bus HB toprovide additional caching capabilities to improve performance of thecomputer systems. The L2 cache 108 may be permanently installed or maybe removable if desired. A cache and memory controller and PCI bridgechip 110, such as the 82424 or 82434X chip from Intel Corporation or thechip described in patent applications Ser. Nos. 08/324,016, entitled"SINGLE BANK, MULTIPLE WAY CACHE MEMORY" and 08/324,246, entitled"MEMORY CONTROLLER WITH WRITE POSTING QUEUES FOR PROCESSOR AND I/O BUSOPERATIONS AND ORDERING LOGIC FOR CONTROLLING THE QUEUES", filed Oct.14, 1994 and hereby incorporated by reference, is connected to thecontrol and address portions of the PCI bus P. The bridge chip 110 isconnected to the L2 cache 108 as it incorporates the cache controllerand therefore controls the operation of the cache memory devices in theL2 cache 108. The bridge chip 110 is also connected to control a seriesof data buffers 112. The data buffers 112 are preferably similar to the82433LX from Intel, or those described in patent applications Ser. Nos.08/324,246 as incorporated above and 08/323,263 entitled "DATA ERRORDETECTION AND CORRECTION SYSTEM," filed Oct. 14, 1994 and now allowedand hereby incorporated by reference, and are utilized to handle memorydata to a main memory array 114. The data buffers 112 are connected tothe processor data portion 102 and receive control signals from thebridge chip 110. The data buffers 112 are also connected to the PCI busP for data transfer over that bus. The data buffers 112 provide a memorydata bus 118 to the memory array 114, while a memory address and memorycontrol signal bus MB is provided from the bridge chip 110.

A video controller 300 is connected to the PCI bus P. Video memory 304is used to store the graphics data and is connected to the videographics controller 300 and a digital/analog converter (RAMDAC) 306. Thevideo graphics controller 300 controls the operation of the video memory304, allowing data to be written and retrieved as required. A videoconnector 308 is connected to the RAMDAC 306. A monitor (not shown) isconnected to the video connector 308.

A network interface (NIC) controller 120 is connected to the PCI bus P.Preferably the controller 120 is a single integrated circuit andincludes the capabilities necessary to act as a PCI bus master and slaveand the circuitry to act as an Ethernet interface. Alternate Ethernetconnectors 124 are provided on the system S and are connected to filterand transformer circuitry 126, which in turn is connected to thecontroller 120. This forms a network or Ethernet connection forconnecting the system boards and computer to a local area network (LAN).

A PCI-ISA bridge 130 is provided to convert signals between the PCI busP and the ISA bus I. The PCI-ISA bridge 130 includes the necessaryaddress and data buffers and latches, arbitration and bus master controllogic for the PCI bus, ISA arbitration circuitry, an ISA bus controlleras conventionally used in ISA systems, an enhanced DMA controllerpreferably having two channels for interfacing with primary andsecondary IDE devices through connectors 133, and an 8237-compatible DMAcontroller. Preferably the PCI-ISA bridge 130 is a single integratedcircuit, but other combinations are possible. To reduce the number ofpins required for the PCI-ISA bridge 130, the EDMA controller 204 (FIG.2) shares the ISA SD or data bus and the upper portion of the SA oraddress bus to perform data transfers between the IDE devices and themain memory 114.

A series of ISA slots 134 (FIG. 1) are connected to the ISA bus I toreceive ISA adapter cards. A series of IDE slots 133 are connected tothe ISA bus I and the PCI-ISA bridge chip 130 to receive various IDEdevices, such as hard disk drives, tape drives and CD-ROM drives. Aseries of PCI slots 135 are connected to the PCI bus P to receive PCIadapter cards.

A combination I/O chip 136 is connected to the ISA bus I. Thecombination I/O chip 136 preferably includes a floppy disk controller,real time clock (RTC), CMOS memory, two UARTs, and various addressdecode logic. A floppy disk connector 138 for receiving a cable to afloppy disk drive is connected to the combination I/O chip 136 and theISA bus I. Serial port connectors 137 are also connected to thecombination I/O chip 136. A data buffer 144 is connected to the address,data and control portions of the ISA bus I to provide an additional Xbus for various additional components of the computer system. A flashROM 154 receives its control, data and address signals from the X busfor data transfer.

Preferably the flash ROM 154 contains the BIOS information for thecomputer system and can be reprogrammed to allow for revisions of theBIOS. An 8042 or keyboard controller 156 is connected to the X bus X.The keyboard controller 156 is of conventional design and is connectedin turn to a keyboard connector 158 and a mouse or pointing deviceconnector 160.

A miscellaneous system logic chip 132 is connected to the X bus X. Themiscellaneous system logic chip 132 contains counters and timers asconventionally present in personal computer systems, an interruptcontroller for both the PCI and ISA buses P and I, enhanced parallelport circuitry and power management logic, as well as othermiscellaneous circuitry.

This is an exemplary computer system S and other variations couldreadily be developed by one skilled in the art.

Referring now to FIG. 2, various control blocks of the PCI-ISA bridge130 are shown. The PCI-ISA bridge 130 includes a PCI interface 207,which consists of PCI master logic 207 and a PCI slave logic 208. ThePCI slave logic 208 is responsible for monitoring the cycles on the PCIbus P and determining when to respond to these cycles. The PCI-ISAbridge 130 is the subtractive decode agent on the PCI bus, i.e., itresponds as a PCI target by asserting a signal DEVSEL* when no other PCIagent responds to the cycle. As the target, the PCI-ISA bridge 130passes the PCI cycles to the ISA bus I. The PCI slave 208 also respondsas a PCI target when it decodes cycles to internal PCI configurationregisters, I/O registers or interrupt acknowledge I/O registers.

The PCI master logic 206 is responsible for running cycles on the PCIbus P on behalf of ISA bus masters, a DMA controller 216, and theenhanced DMA (EDMA) controller 204. On the PCI bus P, the PCI masterlogic 206 runs memory and I/O read and write cycles. The PCI masterlogic 206 also receives a request from the DMA controller 216 for thePCI bus P and in response asserts a signal EREQ₋₋ to a PCI arbiter 210.Similarly the PCI master logic 206 generates EDMAREQ₋₋ to the PCIarbiter 210 in response to an IDE request from the EDMA controller 204.

The enhanced DMA or EDMA controller 204 includes state machines 250, 252and 254 which are described below in FIGS. 22, 24 and 25. The EDMAcontroller 204 also provides two channels in the preferred embodimentfor supporting primary IDE devices 230 and secondary IDE devices 232connected through connectors 133. Each channel is capable of supportingtwo devices configured as a master and slave. The supportable devicesinclude hard disk drives, CD-ROMs, and tape drives. The IDE devices 230and 232 are connected to a data bus HD, which is coupled throughbi-directional data buffers 234 to the SD bus.

The control signals between the IDE devices 230 and 232 and the EDMAcontroller 204 are signals IDE₋₋ WR₋₋, IDE₋₋ RD₋₋, IDE₋₋ DRQ₋₋ P, IDE₋₋DRQ₋₋ S, IDE₋₋ DAK₋₋ P₋₋ and IDE₋₋ DAK₋₋ S₋₋. The write command strobeIDE₋₋ WR₋₋ is asserted by the EDMA controller 204 while a control byteor data word is driven into the selected IDE device, and the commandstrobe IDE₋₋ RD₋₋ is asserted when a status byte or data word isretrieved from the selected IDE device. To begin a transfer operationwith an IDE device, the CPU 100 generates a parallel I/O command cycleon the PCI bus P. The parallel I/O command is typically a read sectorcommand or a write sector command. The I/O command is transmitted to theISA bus and then to the selected IDE device. Each sector commandinvolves the transfer of 256 bytes of data. The CPU 100 then generates asecond PCI I/O cycle to write appropriate IDE control registers in theEDMA controller 204. There are two IDE control registers in the EDMAcontroller 204, one for the primary channel and one for the secondarychannel. In the preferred embodiment, the bits written in each controlregister are the mask bit and the write/read bit. The mask bit indicatesif the channel is masked, and the write/read bit indicates a write orread operation. The mask bits and write/read bits are described below inFIG. 23A. When the selected IDE device is ready to begin the transfer,which usually takes a relatively long time because of seek times, itasserts either of request signals IDE₋₋ DRQ₋₋ P or IDE₋₋ DRQ₋₋ Sdepending on if the primary or secondary channel is selected. Inresponse, the EDMA controller 204 generates a request for the SD bus.Once the SD bus has been granted to the EDMA controller 204, theappropriate one of the acknowledge signals IDE₋₋ DAK₋₋ P₋₋ and IDE₋₋DAK₋₋ S₋₋ is asserted by the EDMA controller 204. Interrupt requestsfrom the IDE devices 230 and 232 are referred to as IRQ₋₋ P and IRQ₋₋ Sand are provided to the miscellaneous system logic chip 132.

Two levels of four-double-word write gathering buffers 209A and 209B andone level of four-double-word read buffers 211 are provided in thePCI-ISA bridge 130 for data transfers between the SD and PCI buses.During reads of the IDE devices (EDMA writes to main memory 114), theEDMA controller 204 initiates a request for the PCI bus as soon as thefirst level four-double-word write buffer 209A is full. The data in thefirst level buffer 209A is then latched into the second level buffer209B. Data transfer from the IDE devices continues until the first levelis again full. This provides for a large latency period for the PCImaster logic 206 to obtain control of the PCI bus P. During writes tothe IDE devices (or EDMA reads of the main memory 114), the EDMAcontroller 204 must pause to obtain the next four-double-word of dataupon completion of a read transfer as there is only one stage of readbuffers 211. Only one level of read buffers is used in the preferredembodiment to reduce complexity in the design of the EDMA controller204. As there are more disk reads than disk writes, with a variance ofalmost a 10-to-1 ratio, the benefits conferred by using twofour-double-word read buffers would be relatively small.

As a result of the above features, it has been found that use of theEDMA controller 204 according to the preferred embodiment reduces PCIbandwidth utilization of up to 95% versus conventional 8237 DMA (withtype B timing) controllers for IDE data transfers.

For added flexibility, the timing for completing a data transfer, i.e.,one word out of the 256 bytes of data in a sector, by the EDMAcontroller 204 between the SD and PCI buses is also completelyprogrammable, as fully described below. The EDMA timing is programmablevia configuration I/O registers in the PCI-ISA bridge 130. This providesadded advantages over conventional 8237 DMA controllers, which allow forlimited timing selectibility between the ISA-compatible timing mode,Type A timing mode, Type B timing mode, or Type C timing (or burst)mode.

The DMA controller 216 in the PCI-ISA bridge 130 includes sevenchannels, each providing 24 bits of memory address. The DMA controller216 presents an 8-bit interface and is programmed with 8-bit I/Oinstructions. The DMA controller 216 supports 8 or 16-bit DMA transfersto memory on the PCI bus P and responds only to I/O devices on the ISAbus I. The DMA controller 216 is effectively equivalent to theconventional chained 8237 pair used in conventional EISA computersystems.

The DMA controller 216 interfaces with an ISA bus controller 214, alsoon the PCI-ISA bridge 130 in the preferred embodiment. The ISA buscontroller 214 includes a refresh controller 215 for running refreshcycles on the ISA bus I. The DMA interface in the ISA bus controller 214translates status signals provided by the DMA controller 216 intoappropriate bus cycles. The ISA bus controller 214 also includes otherlogic blocks, including the PCI bus interface logic, address convertingand sequencing logic, data buffer control logic, and ISA bus masterinterface logic. During cycles on the ISA bus I controlled by an ISA busmaster, the ISA bus controller 214 interprets control signals on the ISAbus I for transmission to the PCI bus P. The ISA bus controller 214 alsogenerates ISA control signals during CPU and DMA cycles. During PCIcycles, the ISA bus interface logic interprets signals from the PCI businterface for driving onto the ISA bus I.

The ISA bus interface logic determines the type of cycle being run fromthree signals: EGNT₋₋ which is driven by the PCI arbiter 210 to indicatethat the ISA bus I has control of the PCI bus P;, EMAST16₋₋ whichindicates a 16-bit ISA master has control of the ISA bus I; andREFRESH₋₋ which indicates a refresh cycle. If the signal EGNT₋₋ isdeasserted, then that indicates a PCI-to-ISA cycle. If the signalsEGNT₋₋ and EMAST16₋₋ are asserted low and the signal REFRESH₋₋ isdeasserted high, then that indicates a 16-bit ISA bus master has controlof the ISA bus I. If the signal EGNT₋₋ is asserted and the signalsREFRESH₋₋ and EMAST16₋₋ are deasserted, then control of the ISA bus I isin the DMA controller 216. If the signals EGNT₋₋ and REFRESH₋₋ areasserted, then an ISA bus master has control of the ISA bus I and isproviding a refresh cycle. If the signals EGNT₋₋ is deasserted andREFRESH₋₋ is asserted, then the refresh controller 215 has control ofthe ISA bus I. It is noted this cycle is not propagated to the PCI bus Pas refreshes on the ISA bus I by the refresh controller 215 are hidden.

The arbitration scheme in the PCI-ISA bridge 130 is divided among threelogic blocks: the PCI arbiter 210, an SD arbiter 212, and an arbiter 218in the DMA controller 216. The PCI arbiter 210 preferably supports atotal of seven masters: five external masters and two internal masters.The external masters include four general purpose external PCI masters,such as the video controller 300 and NIC 120, and the bridge chip 110.The two internal masters are 1) the DMA controller 216 and 2) the EDMAcontroller 204. The PCI arbiter 210 implements a least-recently-used orLRU prioritization scheme. The PCI-ISA bridge 130 itself does not haveany higher priority than any of the other PCI masters--its priority isdetermined solely by the LRU algorithm except when the signal LOCK₋₋ isasserted and a lock cycle is in progress on the PCI bus P, the signalEGNT₋₋ is masked until the locked cycle is completed.

As will be described below in more detail, the PCI arbiter 210implements a mechanism to minimize bus thrashing when a PCI master isretried by a target. A target generates a retry to notify the PCI masterthat the target is currently unable to complete the bus transaction. Foradditional details on retry cycles, refer to the PCI Specification.

The SD arbiter 212 determines which device gets control of the SD or ISAdata bus. The SD arbiter 212 works in conjunction with the ISA busarbiter 218A to determine which master gets control of the ISA bus I.Devices that can control the SD bus are PCI masters (for PCI-to-ISAcycles), the refresh controller 215, the EDMA controller 204, the DMAcontroller 216 and 16-bit ISA bus masters. If there are no requests forthe SD bus, the SD arbiter 212 "parks" the PCI slave on the ISA bus I tominimize the latency for PCI-to-ISA cycles. This will be described inmore detail below in FIG. 15. The refresh controller 215 is the secondhighest priority requester in the SD arbiter 212. Granting refreshcycles such a high priority should not impact overall systemperformance, as refresh cycles occur infrequently, i.e., approximatelyevery 15 microseconds. The EDMA controller 204 has the next higherpriority in the SD arbiter 212. The IDE devices connected through IDEconnectors 133 use the SD bus for transferring the data.

However, the EDMA controller 204 can be preempted off the SD bus in oneof two ways. The first is referred to as bus master preemption, whichoccurs when a refresh or DMA request is pending. If the EDMA controller204 is bus master preempted, the request of the EDNA controller 204 isdeasserted after a page miss has been reached to allow the refreshcontroller 215, DMA controller 216, or ISA I/O device to gain control ofthe bus. The second way of preempting the EDMA controller 204 isreferred to as slave preemption, which occurs when a PCI master needs torun a cycle to the ISA bus I. When this happens, the EDMA controller 204completes its current data transfer before it gives up the SD Bus.However, the EDMA controller 204 asserts a signal BLK₋₋ MASK to indicatethat it was slave preempted and wants access to the ISA bus I after thePCI-to-ISA cycle is completed. The signal BLK₋₋ MASK prevents the IDErequest for the SD bus from being masked by a DMA or refresh controllerrequest.

The device having the lowest priority for access to the SD bus is theDMA controller 216 or ISA I/O device. The DMA controller 216 gainscontrol of the SD bus in one of two situations. First, it may requestcontrol for running DMA cycles. In the preferred embodiment, the DMAcontroller 216 may also request control of the SD bus if an ISA busmaster needs to run a cycle on the ISA bus I. This is accomplished byprogramming the DMA channel assigned to the 16-bit ISA I/O device incascade mode. Thus, a granted request by the I/O device will cause theDMA controller 216 to disable its outputs (except the request andacknowledge signals to the I/O device) to enable the I/O device to gaincontrol of the bus. The DMA request is masked if there is any EDMA datain the SD-to-PCI write posting buffers 209A or 209B. This allows theEDMA data to avoid the two microsecond time-out on the ISA bus I oncethe DMA controller 216 or ISA bus master gains access to the ISA bus I.This also greatly simplifies the logic in the PCI interface logic 207 asit is concerned only with one source of data at a time.

The other arbiter in the arbitration scheme according to the presentinvention is the arbiter 218 in the DMA controller 216. The arbiter 218determines which type of device has priority in the next ISA bus cycle.In the preferred embodiment, the devices that are capable of requestingcontrol of the ISA bus I are the six PCI masters, the DMA controller214, seven ISA bus masters via the DMA controller 214, and the refreshcontroller 215. The arbiter 218 implements a two-way "ping-pong" oralternating priority scheme which has two requester types. The firstrequestor type includes the DMA controller 214 and ISA bus masters,while the second requester type includes the other devices. Once thefirst requestor type gains control of the ISA bus I, it becomes thelowest priority for the subsequent arbitration cycle. This forces theDMA controller 216 or ISA bus master to give up control of the ISA bus Ito allow access by the refresh controller 215, PCI bus masters and EDMAcontroller 204.

If the DMA controller 216 is granted priority in the current arbitrationcycle, the DMA arbiter 218 then performs an arbitration between theseven channels of the DMA controller 216 to determine which DMA channelwins. In the preferred embodiment, each of the seven channels can beprogrammed to cascade mode to allow 16-bit ISA bus masters to go throughthe DMA controller 216 to request control of the ISA bus I.

The DMA arbiter 218 in the 8237-compatible DMA controller 216 determinesthe priority of its channels based on either the fixed or rotatingpriority scheme, as is known to those skilled in the art. For moreinformation on 8237-compatible DMA controllers, refer to PeripheralComponents, Intel Corp., pp. 5-4 to 5-21 (1994), which is herebyincorporated by reference.

Referring now to FIG. 3, the PCI arbiter 210 is illustrated in greaterdetail. Seven request signals, CPUREQ₋₋, EREQ₋₋, EDMAREQ₋₋ and REQ₋₋3:0! are provided to request mask logic 400. The signal CPUREQ₋₋ is therequest from the bridge chip 110 to indicate a request from the CPU 100,the signal EREQ₋₋ is the request from one of the ISA bus devices, thesignal EDMAREQ₋₋ is the request from the EDMA controller 204, and thesignals REQ₋₋ 3:0! are the requests from other PCI masters, includingthe video controller 300 and NIC 120. In this description, a signalmnemonic ending in an underline or an asterisk indicates that it is anactive low signal, while an exclamation point indicates an invertedsignal.

The request mask logic 400 produces signals REQ 6:0! from the signalsCPUREQ₋₋, EREQ₋₋, EDMAREQ₋₋, and REQ₋₋ 3:0!. The signals REQ 6:0! areprovided to a LRU type arbiter 402. The REQ₋₋ 6:0! signals correspond torequest signals EDMAREQ₋₋, REQ 5:2!, EREQ₋₋, and CPUREQ₋₋, respectively.The output of the arbiter 402 is a series of signals referred to as theGNT 6:0! and SGNT 6:0! signals. The GNT 6:0! signals are used todevelop, respectively, the EDMAGNT₋₋, GNT₋₋ 5:2!, EGNT₋₋, and CPUGNT₋₋signals which are respectively the responses to the REQ 6:0! requestsignals provided to the request mask logic 400. The signal GNT 0! isdesignated for the CPU bridge chip 110. The signal GNT 1! is designatedfor the ISA bus I and is driven internally in the PCI-ISA bridge 130.The signals GNT 5:2! are designated as general purpose grant lines andcan be used in whichever configuration as desired. The signal GNT 6! isdriven internally in the PCI-ISA bridge 130 and is designated for theEDMA controller 204.

The SGNT 6:0! signals are the synchronized versions of the GNT signals,that is, they have been latched by a series of D-type flip-flops clockedon the PCICLK signal of the PCI bus P. The GNT 6:0! and SGNT 6:0!signals are provided to PCI status decode logic 404, which also receivesPCI control signals from the PCI bus P. Miscellaneous PCI cycle statussignals are provided by the status decode logic 404. The SGNT signalsare also provided to reservation and mask logic generally referred to as406. As illustrated, the reservation and mask logic 406 includes twoportions, a cycle timer 408, which receives two bits from an arbitraryI/O port, and reservation and mask generation logic 410, which receivessignals ISC₋₋ RTRY₋₋ MASK₋₋ EN and CLOCK₋₋ SLOW₋₋ MASK. The output ofthe reservation and mask generation logic 410 is seven signals referredto as the MASK 6:0! signals, the priority masking signals, and fivesignals referred to as the LOCKED 5:2,0! signals, which indicate whichparticular PCI bus master has locked the PCI bus P. Signals LOCKED 6!and LOCKED 1! are not provided as the EDMA controller 204 and the ISAdevices are not capable of running locked cycles on the PCI bus P.Additionally, a signal referred to as RETRY₋₋ MSTR or retry master isprovided to indicate that a master has been disconnected and a retrycycle has occurred.

The MASK signals are provided to the request mask logic 400, while theLOCKED signals and the RETRY₋₋ MSTR signal are provided to the LRU-typearbiter 402. In addition, the LRU-type arbiter 402 receives the EREQ₋₋signal to determine if an ISA bus request is active. The EREQ₋₋ signalis also provided to a new grant state machine 412, which is utilized toindicate when a new master can be granted control of the PCI bus P.

Certain timers are associated with the grant phase, including theminimum grant timer 414 and a grant timeout timer 416. Twenty-four bitsof I/O from arbitrary ports are connected to the minimum grant timer414, which also receives the GNT 0,2:6! signals and signals referred toas MIN₋₋ GNT₋₋ TMR₋₋ STRT, MIN₋₋ GNT₋₋ TMR₋₋ RST or minimum grant timerstart and reset signals. The minimum grant timer 414 also receives 24I/O bits MIN₋₋ GNT 15:0! and ADD₋₋ MIN₋₋ GNT 7:0!, which determine theminimum grant times for the various PCI bus masters. The minimum granttimer 414 produces two output signals referred to as the MIN₋₋ TMR₋₋ TOsignal and the MIN₋₋ GNT₋₋ TO signals. Both of the signals indicate thatthe minimum grant timer 414 has timed out and that a new grant canoccur.

The minimum grant timer 414 extends the normal grant time of arequesting PCI bus master by a given programmed value, which isdetermined from configuration registers written during power up. Thetimer 414 is shared between grant lines GNT 6:2,0!. For grant lines GNT0,3,4!, the maximum programmable time is 54 PCICLK clocks. For grantlines GNT 2,5,6!, the maximum programmable time is 248 PCICLK clocks. Itis noted that the minimum grant timer 214 is not associated with thegrant line GNT 1! as the ISA bus I is not subject to the minimum granttime.

The output of the grant timeout timer 416 is the GNT16₋₋ TO signal andis provided to indicate that an arbitration should occur as a sufficientperiod, preferably 16 PCICLK clocks, has elapsed without a bus masterstarting activity. In addition, the LRU-type arbiter 402 produces asignal referred to as the GNTS₋₋ MINTO signal to the new grant statemachine 412 for reasons to be detailed below. The various blocks aredetailed in the following description.

The PCI arbiter 210 also includes a flush request block 418 whichgenerates a signal ARB₋₋ FLUSHREQ. The signal ARB₋₋ FLUSHREQ isgenerated for grant lines GNT 2:6!, which as noted earlier are thegeneral purpose grant lines and the internal grant line for the EDMAcontroller 204. If the minimum grant time for a particular grant lineGNT x! is not assigned to value 0 and the corresponding request signalREQ x! is asserted, the signal ARB₋₋ FLUSHREQ is asserted to drive aflush request signal FLUSHREQ₋₋ to the bridge chip 110. Similarly, anISA request indicated by DHOLD will also cause the flush request signalFLUSHREQ₋₋ to be asserted.

When the CPU 100 posts a write to a CPU-to-PCI queue in the bridge chip110, or when the CPU 100 has locked a particular device on the PCI busP, it has been found that the bridge chip 110 would continuously retrycycles to a PCI master even if no data coherency problems exist. Thisproblem exists for the Intel 82424 and 82434 chips, as well as for thebridge chip described in U.S. patent Ser. Nos. 08/324,016 and08/324,246. If the PCI master was assigned a non-zero minimum granttime, then there would be a period of inactivity until the minimum granttimer 414 times out. This degraded overall performance of the PCI bus P.To prevent this problem from occurring, the PCI-ISA bridge 130 assertsthe FLUSHREQ₋₋ signal to flush the CPU-to-PCI write queue and to preventfurther posting to the write queue.

The flush request block 418 receives signals REQ 6:2!, SREQ 6:2!, GNT6:2!, MIN₋₋ GNT 15:0!, ADD₋₋ MIN₋₋ GNT 7:0!, MIN₋₋ GNT₋₋ TO, and ARB₋₋FLUSHREQ₋₋ EN. The signals SREQ 6:2! are provided by synchronizing logic420, which synchronizes the REQ 6:0! signals to the positive edge ofPCICLK to generate the SREQ 6:0! signals. The SREQ 6:0! signals are alsoprovided to the LRU-type arbiter 402. The signal ARB₋₋ FLUSHREQ₋₋ EN isprovided by a configuration register, which if set high enables theARB₋₋ FLUSHREQ signal.

The PCI arbiter 210 also includes a first retry state machine 422. If acycle requested by a PCI master is retried by a target on the firstcycle, the minimum grant timer 414 is cleared to remove the PCI masterfrom the PCI bus P. This optimizes overall bus performance as it islikely in this case that the target will take a while to remove whatevercondition made the target unable to respond to the master. However, ifthe target asserts retry in the second or later cycles, then the minimumgrant timer 414 remains enabled as it is likely the target will be ableto respond within a few PCICLK clocks.

Referring now to FIG. 4, the reservation and mask logic 406 isillustrated. The status decode block 404 receives the PCI status signalsand provides four signals, the SET₋₋ OWNER, CLR₋₋ OWNER, SET₋₋ LOCK andCLR₋₋ LOCK signals. Development of these signals is shown in FIG. 5A, towhich reference is now made. A signal referred to as the FRAME signal isprovided as one input to a two-input AND gate 425. The FRAME signal isinverted from the PCI FRAME₋₋ signal, which indicates when asserted by aPCI master that a bus transaction is beginning and when deasserted thata transaction is concluded. The second input is the |SFRAME signal,which is a synchronized and inverted version of the FRAME signal. Theoutput of the AND gate 425 is the SET₋₋ OWNER signal.

The CLR₋₋ OWNER signal is equated to a signal referred to as PCI₋₋ IDLEwhich is provided as the output of a two-input NOR gate 427. The inputsof the NOR gate 427 are the FRAME and IRDY signals. The IRDY signal isinverted from the PCI signal IRDY₋₋, which indicates when asserted a busmaster's ability to complete the data phase of the current transaction.The signals FRAME and IRDY being driven low indicate that the currentbus master has concluded the transaction on the PCI bus P and that thePCI bus P is idle. The SET₋₋ LOCK signal is provided as the output of athree-input AND gate 424, whose inputs are the IRDY signal, the TRDYsignal and the PCILOCK signal. The TRDY signal is inverted from the PCITRDY₋₋ signal, and indicates when asserted that the target is able tocomplete the current data phase of the transaction. The signal PCILOCKis inverted from the PCI LOCK₋₋ signal, which indicates that an atomicoperation may require multiple transactions to complete, e.g.,read-modify-write operation. The CLR₋₋ LOCK signal is provided at theoutput of a two-input NOR gate 426, whose inputs are the FRAME andPCILOCK signals.

The SGNT 6:0! signals and the SET₋₋ OWNER and CLR₋₋ OWNER signals areprovided as inputs to a bus owner detect logic 428 (FIG. 4). The busowner detect logic 428 is used to provide a series of latched outputsignals referred to as OWNER 6:0!, which refer to the owner of the PCIbus P. Detailed logic in the bus owner detect circuitry 428 isillustrated in FIG. 5A. The SET₋₋ OWNER signal is provided as one inputto a two-input NAND gate 430, whose second input receives the SGNT x!signal, x being equal to 0-6 for the seven grant lines. Only one exampleor channel of the logic is illustrated, the remaining portions beingduplicated. This illustration of a single channel for exemplary purposesis utilized whenever possible in this description.

The output of the NAND gate 430 is provided as one input to a two-inputNOR gate 432, whose second input receives the CLR₋₋ OWNER signal. Theoutput of the NOR gate 432 is provided to the D input of a D-typeflip-flop 434, which receives the PCI₋₋ RESET signal, that is the resetsignal on the PCI bus P, at its clear input and produces the OWNER x!signal at its noninverting output. The flip-flop 434 is clocked by thePCICLK signal. It is noted in FIG. 5A that no connection is shown to theclock input of the flip-flop 434 and this is used uniformly throughoutthe drawings to indicate that the clocking input is the PCICLK signal.Where it is otherwise, a signal is provided to the clocking input of theparticular flip-flop. The various PCICLK signal connections to theflip-flops have been omitted for clarity.

The OWNER 5:2,0!, SET₋₋ LOCK and CLR₋₋ LOCK signals are provided asinputs to lock owner detect logic 436 (FIG. 4). The output of the lockedowner detect logic 436 is the LOCKED 5:2,0! signals. Detaileddevelopment of the lock owner detect circuitry 436 is illustrated inFIG. 5A. The OWNER x! signal is provided as one input to a three-inputNAND gate 438. The second input to the NAND gate 438 is the SET₋₋ LOCKsignal, while the third input is the |LOCK₋₋ ACTV or not lock activesignal. The |LOCK₋₋ ACTV signal is produced at the output of afive-input NOR gate 440, which receives the signals LOCKED 5:2,0!. Theoutput of the NAND gate 438 is provided as one input to a two-input NORgate 442, whose second input receives the CLR₋₋ LOCK signal. The outputof the NOR gate 442 is provided to the D input of a D-type flip-flop444, which is cleared by the PCI₋₋ RESET signal. The non-invertingoutput of the flip-flop 444 is the LOCK x! signal, while the invertedoutput provides the |LOCK x! signal, where x equals 0 and 2-5.

The PCI status signals are also provided to a retry flag state machine450 (FIG. 4). The retry flag state machine 450 is clocked by the PCICLKsignal. The outputs of the retry flag state machine 450 are signalsRETRY₋₋ MSTR signal, ISC₋₋ RETRY₋₋ MSTR, and CPU₋₋ RETRY₋₋ MSTR. TheISC₋₋ RETRY₋₋ MSTR signal is used to indicate that a retry has beenissued by the PCI-ISA bridge 130 and that the cycle which has beenaborted was addressed to either the bridge 130 itself or to the ISA busI. the conditions of this retry are further described below. The CPU₋₋RETRY₋₋ MSTR signal indicates a retry initiated by a PCI master otherthan the PCI-ISA bridge 130. The ISC₋₋ RETRY₋₋ MSTR signal is used tomask off the bus request of the particular master which was retried bythe PCI-ISA bridge 130 until the cycle can be run without a need for afurther retry, while the CPU₋₋ RETRY₋₋ MSTR signal is used to mask offthe request from the bus owner where the target is a device other thanthe PCI-ISA bridge 130. This allows the device that initiated the retryan opportunity to complete the condition that caused the retry. Thesignal RETRY₋₋ MSTR is asserted high if either of the ISC RETRY₋₋ MSTRor CPU₋₋ RETRY₋₋ MSTR signals is high.

The retry flag state machine 450 is illustrated in FIG. 6. Operation ofthe state machine commences at state A upon receipt of the PCI₋₋ RESETsignal. When the FRAME signal is asserted, indicating that a PCI cycleis active, control proceeds to state B. Otherwise, control remains atstate A. If the PCI₋₋ IDLE signal is asserted in state B, indicatingthat the PCI bus P has gone idle, control returns to state A.

For the ensuing discussion, a signal STOP is defined as being theinverted state of the PCI STOP₋₋ signal, which indicates when activethat a target is requesting the current master to stop bus transaction.A signal ODEVSEL is generated in the PCI-ISA bridge 130, and itsinverted state drives the PCI DEVSEL₋₋ signal. The DEVSEL₋₋ signal isdriven low by a target when the target decodes its address on the PCIbus P in the current cycle.

If the STOP signal is asserted, and the TRDY, ODEVSEL and FRAME signalsare not asserted, control proceeds to state D. This indicates anoperation on the PCI bus P with a bus master other than the PCI-ISAbridge 130. In state D, the signals CPU₋₋ RETRY₋₋ MSTR and RETRY₋₋ MSTRare asserted.

If the STOP signal is asserted, the TRDY signal is deasserted, the FRAMEsignal is deasserted, and the ODEVSEL signal is asserted, controlproceeds to state C. This indicates an operation with the PCI-ISA bridge130 as the target, and the PCI-ISA bridge 130 has sent a retry to thebus master. In state C, the signals ISC₋₋ RETRY₋₋ MSTR and RETRY₋₋ MSTRare asserted. In state B, if both signals IRDY and TRDY are asserted,indicating that both the current bus master and target are ready tocomplete the data phase of the current transaction on the PCI bus P,then control proceeds to state E, regardless of the state of the STOPsignal. The state machine remains in state E until the data has beentransferred. When the PCI bus P goes idle, as indicated by the signalPCI₋₋ IDLE, the state machine transitions from state E back to state A.

Similarly, the state machine returns to state A from either state C or Dif the signal PCI-IDLE is detected asserted.

Thus the state machine 450 indicates when a cycle has been retried andwhether it was directed to the ISA bus I or a device under control ofthe PCI-ISA bridge 130, or to a PCI bus target other than the PCI-ISAbridge 130.

The OWNER 6:0! and LOCKED 5:2,0! signals and the ISC₋₋ RETRY₋₋ MSTRsignal are provided to the mask generation logic 452 (FIG. 4), whichalso receives a signal referred to as the ISC₋₋ RETRY₋₋ MSK₋₋ EN or ISCretry mask enable signal from a bit in an arbitrary I/O port. This bitis used to enable or disable operation of the feature wherein the busrequest signal of a requesting bus master which has been retried basedon an access to the PCI-ISA bridge 130 is masked until the cycle can becompleted without a further retry. When this feature is disabled, whichis not preferable, then the master may repeatedly retry prior to thedata being available. The output of the mask generation logic 452 is theISC₋₋ MSK 5:2,0! signals. As the PCI-ISA bridge 130 will not initiate aretry in a cycle that it generated, there exists no need for signalsISC₋₋ MSK 1! and ISC₋₋ MSK 6!.

Further details of the logic are provided in FIG. 5A. The OWNER x! and|LOCKED x! signals are the inputs to a two-input AND gate 454, where xis equal 0 and 2-5. The output of the AND gate 454 is connected to the Dinput of a D-type flip-flop 456. The non-inverted output of theflip-flop 456 is the ISC₋₋ MSK x! signal. The D flip flop 456 is clockedon the rising edge of the signal ISC₋₋ RETRY₋₋ MSTR. The clear input ofthe flip-flop 456 is connected to the output of an OR gate 460 whichreceives the PCI₋₋ RESET, |RETRY and |ISC₋₋ RETRY₋₋ MSK₋₋ EN signals.The RETRY signal is provided under several conditions. First, a cycle isdirected to the ISA bus I, but another cycle is already in progress onthe ISA bus I. One example is when a prior master has posted a writeoperation to the ISA bus I and that write operation is occurring.Second, a cycle is directed to the ISA bus I when a refresh cycle on theISA bus I is pending or is in progress. The third condition is when thePCI-ISA bridge 130 is the responding PCI slave, a lock has been set andthe requesting bus master is not the locking bus master. This conditionoccurs as the PCI-ISA bridge 130 must not execute a cycle as a lockedresource to any master except the one placing the lock. The finalcondition occurs when the system clock is changing frequency and themaster requesting the PCI bus P is not the CPU 100 (bridge chip 110).The RETRY signal is asserted when any of these events occur and isremoved or negated when the assertion event is completed, such as thelock being released, the posted write completing, the refresh completingor the system clock frequency change completing as indicated by thesignal CLOCK₋₋ SLOW₋₋ MASK being deasserted. The PCI-EISA bridge 130 canobviously determine when it is unlocked and can determine the otherthree events as it performs the posted write operation, includes therefresh controller and monitors the state of the signal CLOCK₋₋ SLOW₋₋MASK. Therefore if a cycle initiated by a bus master and directed to thePCI-ISA bridge 130 is retried, and the PCI₋₋ ISA bridge 130 is notlocked by the bus master, then the corresponding ISC₋₋ MSK bit is set toallow the master's bus request to be masked until the retry source eventis completed as indicated by the signal RETRY being deasserted.

As mentioned above, there are certain conditions when a PCI master isretried when the target is not the PCI-ISA bridge 130. In thissituation, it is desirable to mask the request from the current busowner so that the target device that initiated the retry can completethe condition that caused the retry. The current bus owner is masked offfor a certain period, and it is desirable the masking period beprogrammable. To this end, two bits from an arbitrary I/O port areutilized to define four options. The 00 value indicates that masking isdisabled, while the other three combinations refer to 4, 8 and 16 PCICLKsignal delays. These two bits are provided to the inputs of timing block480 (FIG. 4), which is shown in more detail in FIG. 5B. The timing block480 provides output signals |CNTR₋₋ ACTV, which indicates when true thata CPU timer 470 (FIG. 4) is inactive, and CPU₋₋ TO, which indicates thetime-out condition for the timer 470. The timer 470 is preferably afour-bit timer providing output bits CNTR 3:0! to the logic block 480.If the timeout signal CPU₋₋ TO is asserted, the load input of the timer470 is activated to load the value 0x0000 into the timer 470. The timer470 is clocked on the rising edge of the PCICLK signal, and it isstarted upon receipt of a signal which is provided by the output of afour-input AND gate 472. The inputs to the AND gate 472 are the PCI₋₋IDLE signal, the CPU₋₋ RETRY₋₋ MSTR signal, the |CNTR₋₋ ACTV signal andthe |COUNT₋₋ DISABLED signal. So when the timer 470 is not disabled asindicated by |COUNT₋₋ DISABLED, and not currently active as indicated by|CNTR₋₋ ACTV, and a retry cycle has been initiated as indicated by CPU₋₋RETRY₋₋ MSTR, and the PCI bus P is idle because of the retry, then thetimer 470 is started.

The timer 470 is reset by the output, referred to as the CPU₋₋ TO₋₋ RSTsignal, of a two-input OR gate 474. One input to the OR gate 474 is thePCI₋₋ RESET signal and the other input is provided by the non-invertingoutput of a D-type flip-flop 476. The D input of the flip-flop 476receives the output of a two-input NOR gate 478 which receives at itsinputs the two bits to define the timeout interval. The inverted outputof the flip-flop 476 is the |COUNT₋₋ DISABLED signal.

The timer output bits CNTR 3:0! are provided to the inputs of the logicblock 480 (FIG. 5B), as are the signals PCICLK and CPU₋₋ TO₋₋ RST.Referring now to FIG. 5B, the logic block 480 is shown. A four-input NORgate 494 receives the CNTR 3:0! bits. If all four CNTR 3:0! are low,then the NOR gate 494 drives the signal |CNTR₋₋ ACTV high. The signalCPU₋₋ TO is provided by a D-type flip-flop 499, which is clocked on therising edge of the PCICLK signal. The D input of the flip flop 499 isconnected to the output of an AND gate 498. One input of the AND gate498 is connected to the inverted state of the signal CPU₋₋ TO, while theother input is connected to the output of a 4-to-1 multiplexor 489. Theselect inputs of the multiplexor 489 are connected to the two bits fromthe arbitrary I/O port for selecting the timing interval.

The 1, 2 and 3 inputs of the multiplexor 489 are connected to theoutputs of comparators 495, 496 and 497, respectively. The 0 input is a"don't care" condition. The comparators 495, 496 and 497 compare theCNTR 3:0! bits with the values 0b0010, 0b0110 and 0b1110, respectively.Thus, if the select inputs of the multiplexor 489 are driven with thevalue 0b01, the CPU₋₋ TO signal is asserted when the timer 470 counts to0b0010 to effectively force a masking interval of 4 PCICLK clocks. Ifthe select inputs of the multiplexor 489 are driven with the value 0b10,then the CPU₋₋ TO signal is asserted when the timer 470 counts to0b0110, which effectively forces a masking interval of 8 PCICLK clocks.Finally, if the select inputs of the multiplexor 489 are driven with thevalue 0b11, then the CPU TO signal is asserted when the timer 470reaches the value 0b1110, which effectively forces a masking interval of16 PCICLK clocks.

The CPU₋₋ RETRY₋₋ MSTR, CPU₋₋ TO and |COUNT DISABLED signals areprovided to the CPU mask generation logic 482, which provides the CPU₋₋MSK 6:2,0! signals. It is noted that a CPU₋₋ MSK 1! signal is notprovided as the ISA bus request is not subject to masking in thepreferred embodiment. The CPU mask generation logic 482 further receivessignals LOCKED 5:2,0! and OWNER 6:2,0!. Details of the CPU maskgeneration logic 482 are provided in FIG. 5B for the generation of theCPU₋₋ MSK 5:2,0!; The CPU₋₋ RETRY₋₋ MSTR signal is provided as one inputto a four-input NAND gate 484, with the other inputs receiving the|COUNT₋₋ DISABLED signal, the OWNER x! signal and the |LOCKED x! signal,with x equal to 0 and 2-5. The output of the NAND gate 484 is providedas one input to a two-input NOR gate 486 with the second input receivingthe CPU₋₋ TO signal. The output of the NOR gate 486 is provided to the Dinput of a D-type flip-flop 488, whose non-inverted output provides theCPU₋₋ MSK x! signal. The flip-flop 488 is clocked by the PCICLK signaland is cleared by the PCI₋₋ RESET signal. Thus, the CPU₋₋ MSK x! signalis asserted when the CPU₋₋ RETRY₋₋ MSTR signal is asserted if the cycleinitiated by the bus owner x is not locked and the timer 470 is notdisabled.

The CPU₋₋ MSK 6! signal is provided by a D-type flip-flop 491, which isalso clocked by the PCICLK signal and cleared by the PCI₋₋ RESET signal.The D input of the flip-flop 491 is connected to the output of a NORgate 490, whose first input receives the signal CPU₋₋ TO and whose otherinput is connected to the output of a three-input NAND gate 487. TheNAND gate 487 receives input signals OWNER 6!, CPU₋₋ RETRY₋₋ MSTR, and|COUNT₋₋ DISABLED. Thus, the EDMA controller 204 is masked when it isthe owner of the PCI bus P, the timer 470 is not disabled, and a targetother than the PCI-ISA bridge 130 asserts a retry cycle. Each of theCPU₋₋ MSK 6:2,0! signals are cleared low when the timeout signal CPU₋₋TO is asserted high.

The ISC₋₋ MSK 6:2,0! and CPU₋₋ MSK 6:2,0! signals are provided to maskgeneration logic 490 as is the signal CLOCK₋₋ SLOW₋₋ MASK. The output ofthe mask generation logic 490 is the MSK 6:2,0! signals to indicatewhich bus request signals are to be masked from the actualprioritization process. Details of the circuitry are provided in FIG.5B. The signal MASK 0! is provided by an OR gate 492, which receives theCPU₋₋ MSK 0! and ISC₋₋ MSK 0! signals. The signals MASK x!, x equal to2-5, is provided by an OR gate 493, which receives the signals CPU₋₋ MSKx!, ISC₋₋ MSK x! and CLOCK₋₋ SLOW₋₋ MASK. Thus, while the system clockfrequency is being changed as indicated by the signal CLOCK₋₋ SLOW₋₋MASK, requests from bus masters 2-5 are masked. The signal MASK 6! issimply equated with the signal CPU₋₋ MSK 6!, as there is nocorresponding ISC₋₋ MSK 6! signal.

The operation of the new grant state machine 412 (FIG. 3) is illustratedin FIG. 7, which as explained above tracks the grant and bus activity todetermine when new grants are issued and when current grants areremoved. Control begins at state A upon receipt of the PCI₋₋ RESETsignal. State A is the idle state which indicates that no activity iscurrently occurring on the PCI bus P. If there is an active grant asindicated by the GNT₋₋ ACTV signal, control proceeds to state B. TheGNT₋₋ ACTV signal is produced as a seven-input OR gate 500 (FIG. 12).The seven inputs to the OR gate 500 are the GNT 6:0! signals. The GNT₋₋ACTV signal is also provided as one input to a two-input AND gate 502,whose other input is inverted and receives the GNT 1! signal. The outputof the AND gate 502 is the GNT₋₋ ACTV₋₋ N1 or grant active except forISA grant signal. The SGNT₋₋ ACTV₋₋ N1 signal is the synchronizedversion of the GNT₋₋ ACTV₋₋ N1 signal. Thus, control proceeds to state B(FIG. 7) when the signal GNT₋₋ ACTV₋₋ N1 is asserted and the signalPCI₋₋ IDLE is deasserted.

Control proceeds from state A to state E if the GNT1₋₋ NLCK signal ispresent, indicating that the ISA bus I has control of the PCI bus P andno lock signal is active, as indicated by the LOCK₋₋ ACTV2 signal. TheGNT1₋₋ NLCK signal is provided by the output of a two-input AND gate 508(FIG. 12). The inputs to the AND gate 508 are the GNT 1! signal and the|LOCK₋₋ ACTV signal. The LOCK₋₋ ACTV signal is produced by the output ofa six-input OR gate 503 (FIG. 13) whose inputs are the LOCKED 0,2:5!signals. Control proceeds from state A (FIG. 7) to state F if the PARK₋₋CPU signal is asserted and the CPUGNT (GNT 0!) signal is asserted.PARK₋₋ CPU is an indication that no one has requested the bus andtherefore the CPU is given priority as the default owner. In all othercases, control remains at state A.

Summarizing, state B is the active state for all grants except the CPU(when it is indicated as being parked on the PCI bus P) and the ISA busI because of special requirements for the ISA bus I. State E is theactive grant state for the ISA grant. State F is the active grant statefor parking the CPU. New grants are disabled in all three states B, Eand F, and is indicated by a signal NEW₋₋ GNT being deasserted low.

Control returns from state E to state A when the. |EREQ signal is true,that is, when the ISA request signal is not present. Otherwise, controlremains at state E. Thus, once the ISA bus I has gained ownership of thePCI bus P, it remains there until its operation is completed.

Control proceeds from state B back to state A if the GNT16₋₋ TO signalis true or if the GNTS₋₋ MINTO and MIN₋₋ GNT₋₋ TO signals are true. Asindicated above, the GNT16₋₋ TO signal indicates that 16 PCICLK cycleshave elapsed without activity on the bus after a change in ownership,while the GNTS₋₋ MINTO and MIN₋₋ GNT₋₋ TO signal term indicates thatcertain masters having minimum grant times have control of the bus andthe minimum grant timer 414 has timed out. Control proceeds from state Bto state D when the GNTS₋₋ MINTO signal is true, the PCI₋₋ IDLE signalis not asserted and the MIN₋₋ GNT₋₋ TO signal is not asserted. This isan indication that the minimum grant timer 414 is enabled but has notexpired and the bus master is active on the bus so that no grants aredisabled. Control proceeds from state B to state C when the PCI₋₋ IDLEsignal is deasserted and the GNTS₋₋ MINTO signal is deasserted or if thePCI₋₋ IDLE signal is deasserted and the GNTS₋₋ MINTO and MIN₋₋ GNT₋₋ TOsignals are asserted. This indicates that the bus is active and it haseither timed out or is not subject to a minimum grant time. In a casewhich should not normally occur, but which is inserted in case offailure, control proceeds from state B to state E when the GNT1₋₋ NLCKand PCI₋₋ IDLE signals are asserted. Otherwise control remains at stateB.

Control proceeds from state D back to state A if the PCI₋₋ IDLE signalis true and the GNT16₋₋ TO signal is true or if the MIN₋₋ GNT₋₋ TOsignal is true, indicating either a period of sufficient inactivitywithout a bus master taking control of the bus or the minimum granttimer 414 has expired and the bus is idle. Control proceeds from state Dto state C if the bus is not idle as indicated by the |PCI₋₋ IDLEsignal, the MIN₋₋ GNT₋₋ TO signal is true or the GNT16₋₋ TO signal istrue. This is a case when the bus master is still active after theminimum grant time or the bus master has gone idle for 16 PCICLK clocks.Otherwise, control remains at state D.

Control proceeds from state C to state A when the PCI₋₋ IDLE signal isasserted, indicating that the PCI bus P is idle. Control proceeds fromstate C to state E in the improper condition where the GNT1₋₋ NLCK andPCI₋₋ IDLE signals are asserted otherwise control remains at state C.

Control proceeds from state F back to state A if the bus is idle and the|PARK₋₋ CPU signal is asserted indicating that the PCI bus P is nolonger parked with the CPU. Control proceeds from state F to state Cunder two conditions, the first of which is that the |PARK₋₋ CPU signalis true, the |PCI₋₋ IDLE signal is true and the |GNTS₋₋ MINTO signal istrue. This term is used when the CPU is not to be the default master,the bus is not idle and a bus master with the minimum grant timer 414enabled is not in control. The second condition for the transfer is ifthe |PARK₋₋ CPU signal is true, the |PCI₋₋ IDLE signal is true, theGNTS₋₋ MINTO signal is true and the MIN₋₋ GNT₋₋ TO signal is true,indicating that the minimum grant that has elapsed and the bus is notidle. In all other cases control remains at state F.

FIG. 8 illustrates logic for generating the outputs of the new grantstate machine 412. A NEW₋₋ GNT signal is provided as the output of athree-input AND gate 504. The MIN₋₋ GNT TO signal and the |GNT1₋₋ NLCKsignals are two inputs to the AND gate 504, while the third input isprovided by the output of a two-input OR gate 506. The inputs to the ORgate 506 are signals that indicate that the next state of the new grantstate machine 412 will be state A or state C. Thus the NEW₋₋ GNT signalis Active when the ISA grant or lock is not active, and either the bushas gone idle or the minimum grant timer 414 (FIG. 3), if any, hasexpired. An EGNT₋₋ EN or ISA grant enable signal is provided as theoutput of a two-input AND gate 510 (FIG. 8), one of the inputs receivingthe GNT1₋₋ NLCK signal. The other input is provided by the output of atwo-input OR gate 512, whose inputs indicate that the state machine isin state A or state E. Thus the EGNT₋₋ EN signal is active when eitherthe bus is idle or the ISA bus I is in control of the PCI bus P. TheEGNT₋₋ EN is used to enable the EGNT₋₋ signal, which is the ISA grantline. The final output of the new grant state machine 412 is a MNGNT₋₋CLR signal, which is provided as the output of a two-input AND gate 514whose inputs receive a signal that indicates that the state machine isin state D and the MIN₋₋ GNT₋₋ TO signal The MNGNT₋₋ CLR signal is onecondition for clearing the GNT 6:0! lines.

The minimum grant timer 414 (FIG. 3) is designed so that particulardevices, in the preferred embodiment, particularly the CPU, the videocontroller 300, the network interface controller (NIC) 120, the EDNAcontroller 204, and other devices as can later be added to the system,can have certain minimum grant or bus access times to allow them to doat least certain minimal operations This is in contrast and differentfrom the GNT₋₋ TO timer 416, which is used to determine if a device hasnot responded within the first 16 PCICLKs after receiving the PCI bus P,in which case mastership is transferred. The MIN₋₋ GNT timer logic 414assures that once the particular device obtains the bus, it has it for aminimum number of PCICLK cycles. To this end twenty-four data bits areprovided to the timer 414 to specify the minimum times for the fourparticular devices in the preferred embodiment. When the minimum granttimer 414 is started, this tire value is loaded into a countdown timerwhich then counts down to zero.

Referring now to FIG. 9, the timer 414, which is clocked by the PCICLKsignal, receives a signal referred to as MIN₋₋ GNT₋₋ TMR₋₋ RST orminimum grant timer reset. This signal is provided as the output of afive-input OR gate 510. The five inputs to the OF gate 510 are the MIN₋₋TMR₋₋ TO signal, which is actually an output of the minimum grant timer414; the GNT16₋₋ TO signal, to indicate that the initial idle bus timehas elapsed; the PARK₋₋ CPU signal, which indicates that the CPU isparked by default on the PCI bus P; an SPARK₋₋ CPU signal, which is thePARK₋₋ CPU signal synchronized to the PCICLK signal; and a CLR₋₋ MIN₋₋GNT₋₋ TO signal, provided by the first retry state machine 422 (FIG. 3)to indicate that a retry was issued in the first cycle, and as a result,the minimum grant timer 414 should be cleared. The first retry statemachine 422 is described below in FIG. 14. When the MIN₋₋ GNT₋₋ TMR₋₋RST signal is high, the minimum grant timer 414 is reset.

The MIN₋₋ GNT₋₋ TMR₋₋ STRT signal is used to start the timer 414, Tothis end, the GNT 6! and |SGNT 6! signals are provided to the two inputsof AND gate 512 (FIG. 7). Similarly, the CNT and |SGNT signals for busmasters 0, 2, 3, 4, and 5 are provided to the inputs of AND gates 513,514, 515, 516, and 517, respectively. The outputs of the AND gates512-517 are provided to the 6 inputs of an OR gate 520, whose output isprovided to one input of a two-input AND gate 521. The other input ofthe AND gate 521 receives the inverted state of the signal SPCI₋₋ IDLE.The output of the AND gate 521 indicates that a new grant has beenissued to a bus master while the PCI bus P was in an active state.

The output of the AND gate 521 is provided to the D input of a D-typeflip flop 522, which is clocked on the PCICLK signal and reset by PCI₋₋RESET. The non-inverting output of the flip flop 522 is connected to oneinput of a two-input oR gate 524, whose other input is connected to theoutput of a two-input AND gate 523. The inputs of the AND gate 523receive the GNT₋₋ ACTV₋₋ N1 and |SGNT₋₋ ACTV₋₋ N1 signals. As notedabove, the signal GNT₋₋ ACTV₋₋ N1 indicates that one of the grant linesGNT 6:0!, other than GNT 1!, has been asserted. The signal |SGNT₋₋ ACTVN| is the inverted version of the GNT₋₋ ACTV₋₋ N1 signal delayed by onePCICLK clock. The output of AND gate 523 indicates also that a new granthas been asserted.

The output of the OR gate 524 is provided to one input of a three-inputAND gate 526, whose other inputs receive the SPCI₋₋ IDLE and |MIN₋₋GNT₋₋ TMR₋₋ RST signals. Thus the timer 414 is started when the PCI busP is idle, the minimum grant timer 414 is not being reset, and a newgrant has been provided to one of the bus masters.

The MIN₋₋ GNT₋₋ TMR₋₋ STRT signal is provided to one input of atwo-input AND gate 528. The second input of the AND gate 528 receives anindication that the timer 414 has been programmed with a minimum counttime of zero for a particular bus master. The output of the AND gate 528is provided to the D input of a D-type flip-flop 530, whose preset inputis connected to the PCI₋₋ RESET signal. The flip flop 530 is presethigh. The non-inverting output of the flip-flop 530 is the MIN₋₋ GNT₋₋DISABLE signal, which is used to stop the timer 414 The inverted outputof the flip-flop 530 is connected to one input of a two-input AND gate532, with the other input receiving the MIN₋₋ GNT₋₋ TMR₋₋ STRT signal.The output of the AND gate 532 is provided to an inverted input of atwo-input AND gate 534, with the other input receiving a signal TMR=0indicating that the timer 414 has counted down to 0. The output of theAND gate 534 is provided to the D input of a D-type flip-flop 536, withthe PCI₋₋ RESET signal being provided to the preset input The output ofthe flip-flop 536 is the MIN₋₋ TO or minimum timeout signal, which isprovided to one input of a two-input AND gate 538 and to one input of afour-input AND gate 540. The second input to the AND gate 538 isinverted and is connected to the output of the AND gate 532. Similarly,the output of the AND gate 532 is connected to an inverted input of theAND gate 540. The inverted output of the flip-flop 530 is connected tothe third input of the AND gate 540, while the non-inverted output of aD-type flip-flop 542 is connected to the fourth input of the flip-flop540. The D input of the flip-flop 542 receives the output of an OR gate544, whose inputs are the |TMR=0 signal, which indicates that the timer414 has not counted down to zero after being loaded, and the output of atwo input AND gate 546, whose inputs are the MIN₋₋ GNT₋₋ TMR₋₋ STRTsignal and the TMR=0 signal.

The output of the AND gate 538 is the MIN₋₋ GNT₋₋ TO signal while theoutput of the AND gate 540 is the MIN₋₋ TMR₋₋ TO signal. Thus, in thismanner, when the MIN₋₋ GNT₋₋ TMR₋₋ STRT signal is received, the timer414 is loaded with the proper value and commences down countingoperations. This continues until the timer 414 reaches zero, at whichtime the MIN₋₋ GNT₋₋ TO signal and the MIN₋₋ TMR₋₋ TO signal areasserted to indicate a timeout. However, if the minimum grant timer 414is programmed with the value zero, then the MIN₋₋ GNT₋₋ TO and MIN₋₋TMR₋₋ TO signals are disabled, as is the timer 414.

It has been noted that the grant timeout timer 416 (FIG. 3) counts a 16PCICLK period when a bus master has ownership but the PCI bus P is idle.The timer 416 is enabled by the output of a five-input AND gate 542(FIG. 9). The inputs to the AND gate 542 are the |GNT 1! signal; theSPCI₋₋ IDLE signal, which is the synchronized version of the PCI₋₋ IDLEsignal; the |PARK₋₋ CPU signal; the |MIN₋₋ TMR₋₋ TO signal; and theGNT₋₋ ACTV signal, which indicates one of the grant lines GNT 6:0! hasbeen asserted. Thus the timer 416 is activated when the minimum granttimer 414 has not timed out, any of the bus masters has been granted thebus, except for the ISA bus I, and the PCI bus P is idle but not bydefault to the CPU. The timer 416 then counts for 16 PCICLK periods andthen issues the GNT16₋₋ To signal to indicate that it has timed out.

FIG. 10 shows a more detailed block diagram of the LRU type arbiterlogic 402. The REQ 6;0! signals are provided to the SYNC₋₋ REQ block 420which contains a series of seven D-type flip-flops clocked by the PCICLKsignal. The SYNC₋₋ REQ block 420 synchronizes the REQ 6:0! signals toproduce the SREQ 6:0! signals. The SREQ 6:0! signals are provided to themodified priority decoder 602 and to the actual grant decoder or arbiter604, as is the signal REQ 1!. Further, the LOCKED 5:2,0! signals areprovided to both the grant decoder 604 and the modified priority decoder602.

A series of priority registers 606 are used to determine the particularpriority of the various masters with respect to each other To this end,signals SNGNT 6:0!, which are the SGNT 6:0! signals synchronized to thenegative edge of the PCICLK signal are provided to priority registers606, as are the RETRY₋₋ MSTR and REARB signals. In addition, the outputsignals of saved priority registers 608 are provided to the priorityregisters 606. The output signals of the priority registers 606 areprovided to the saved priority registers 608 and to the modifiedpriority decode logic 602. The modified priority logic 602 also receivesthe PARK₋₋ CPU signal to be utilized when no master is requesting thePCI bus P.

A more detailed schematic of the modified priority decode logic 602 andthe priority and saved priority registers 606 and 608 is provided inFIG. 11. It is noted that 21 bits are stored by the priority register606 and the saved priority register 608. This corresponds to 1 bit foreach combination of bus masters As an example, 1 bit is provided for busmaster 0 versus bus master 6, one for bus master 0 versus bus master 5and so forth. In the following discussion, this is referred to as bits Xand Y with X being the first bus master and Y being the second busmaster in the particular stored bit. The following description appliesfor X equal to 0-5 and Y equal 1-6. The priority signals P XY! and savedpriority signals SVP XY! are as follows in the preferred embodiment:P01, P02, P03, P04, P05, P06, P12, P13, P14, P15, P16, P23, P24, P25,P26, P34, P35, P36, P45, P46, P56; and SVP01, SVP02, SVP03, SVP04,SVP05, SVP06, SVP12, SVP13, SVP14, SVP15, SVP16, SVP23, SVP24, SVP25,SVP26, SVP34, SVP35, SVP36, SVP45, SVP46, SVP56

The SVP XY! or saved priority XY! signal is provided as one input to atwo-input AND gate 610. The other input is the RETRY₋₋ MSTR signal. Theoutput of the AND gate 610 is provided as one input to a two-input ORgate 612, Whose other input is the PCI₋₋ RESET signal. The output of theOR gate 612 is provided as one input to an OR gate 614, whose otherinput is the SNGNT Y! signal. The output of the OR gate 614 is providedto the preset input of a D-type flip-flop 616. The |SVP XY! signal isprovided as one input to a two-input AND gate 618, with the other inputreceiving the RETRY₋₋ MSTR signal, The output of the AND gate 618 isprovided as one input to a two-input OR gate 620, whose other inputreceives the SNGNT X! signal. The output of the OR gate 620 is providedto the clear input of the flip-flop 616. It is noted that the preset andclear inputs of the flip-flop 616 are synchronous. The clocking signalto the flip-flop 616 is provided by the output of an OR gate 617 whichreceives the REARB signal and the outputs of the OR gates 612 and 618 asinputs.

The non-inverted output of the flip-flop 616 is the P XY! or priority XYbit and this is provided to the D input of the flip-flop 616 and to theD input of a D-type flip-flop 622. The PCI₋₋ RESET signal is provided tothe preset input of the D-type flip-flop 622. The non-inverting outputof the flip-flop 622 is the SVP XY! signal, while the inverted outputprovides the |SVP XY! signal. The flip-flop 622 is clocked by the outputof an OR gate 623, whose inputs are the REARB and PCI₋₋ RESET signals.

For the following discussion, the case of X equal to 1 is excluded. TheP XY! signal is also provided as one input to a two-input OR gate 624,with the second input being the LOCKED X! signal. The output of the ORgate 624 is provided to a two-input AND gate 626, whose second input isthe SREQ X! signal. The output of the AND gate 626 is one input to an ORgate 628, whose output is the MP XY! or modified priority XY signal. The|P XY! signal is provided as one input of a two-input AND gate 630,whose other input is the |SREQ Y! signal. It is noted that the |REQ 1!signal is utilized in the 1 or ISA channel, instead of the |SREQ 1!signal. The output of the AND gate 630 is the second input to the ORgate 628. The third input to the OR gate 628 is the PARK₋₋ CPU signal,in the case of the channels including the CPU, i.e., where X is equal to0, and is not utilized in other channels. The signals provided by the ORgate 628 are MP01, MP02, MP03, MP04, MP05, MP06, MP23, MP24, MP25, MP26,MP34, MP35, MP36, MP45, MP46, and MP56.

For the case of X equal to 1 and Y equal to 2-6, the MP 1Y! signals areprovided by a two-input OR gate 631. The inputs of the OR gate 631 areconnected to the outputs of AND gates 627 and 629. The inputs of ANDgate 627 receive signals P 1Y! and REQ 1!, while the inputs of AND gate629 receive signals |P 1Y! and |SREQ Y!.

In this manner, when a master is retried, the saved priority bit issaved in the flip-flop 616, but if not retried, the master losespriority with respect to all masters. Therefore, should a master have tobe retried, it retains its priority with regard to all other masters, soit can have priority access to regain the bus upon its next request.However, if it is not being retried, then priority is flipped withrespect to its other master. The modified priority decode logic 602,detailed in AND and OR gates 624, 626, 627, 628 629, 630, and 631, isutilized to allow only requesting masters to enter the arbitration. If,for instance, a master has lower absolute priority, i.e. has been usedmore recently then another master, but that second master is notrequesting the bus, the effective priority utilized in the arbitrationis flipped, so that the requesting master has priority over allnon-requesting masters.

The equations for the grant decode logic 604 are shown below.

    ______________________________________    CPU.sub.-- REQ = SREQ0 || PARK.sub.-- CPU    D.sub.-- GNT 0! = MP01 && MP02 && MP03 && MP04 && MP05 &&    MP06 && (|LOCK.sub.-- REQ.sub.-- ACTV || LOCKED 0!) &&    CPU.sub.-- REQ    D.sub.-- GNT 1! = MP12 && MP13 && MP14 && MP15 && MP16 &&    (|MP01 && |LOCK.sub.-- REQ.sub.-- ACTV) && REQ 1!    D.sub.-- GNT 2! = MP23 && MP24 && MP25 && MP26 && (|MP02 &&    |MP12 && |LOCK.sub.-- REQ.sub.-- ACTV || LOCKED 2!) &&    SREQ 2!    D.sub.-- GNT 3! = MP34 && MP35 && MP36 && (|MP03 && |MP13 &&    |MP23 && |LOCK.sub.-- REQ.sub.-- ACTV || LOCKED 3!) &&    SREQ 3!    D.sub.-- GNT 4! = MP45 && MP46 && (|MP04 && |MP14 && |MP24 &&    |MP34 && |LOCK.sub.-- REQ.sub.-- ACTV || LOCKED 4!) &&    SREQ 4!    D.sub.-- GNT 5! = MP56 && (|MP05 && |MP15 && |MP25 && |MP35    && |MP45 && |LOCK.sub.-- REQ.sub.-- ACTV || LOCKED 5!)    &&    SREQ 5!    D.sub.-- GNT 6! = (|MP06 && |MP16 && |MP26 && |MP36 && |MP46    && |MP56 && |LOCK.sub.-- REQ.sub.-- ACTV) && SREQ 6!    ______________________________________

The LOCK₋₋ REQ₋₋ ACTV signal is asserted if a LOCKED signal and thecorresponding SREQ signal is asserted. Thus, the signal LOCK₋₋ REQ₋₋ACTV is asserted if the signals LOCKED 0! and SREQ 0! are asserted, orthe signals LOCKED 2! and SREQ 2! are asserted, or the signals LOCKED 3!and SREQ 3! are asserted, or the signals LOCKED 4! and SREQ 4! areasserted, or the signals LOCKED 5! and SREQ 5! are asserted. Therefore,it can be seen that the particular grant is provided when all of themodified priority bits point to that particular bus master, that busmaster has either locked the bus or no lock requests are active and thatmaster is requesting the bus. As noted above, the LOCKED signal does notexist for bus master 1 or 6 as the ISA bus I and the EDMA controller 204cannot run locked cycles on the PCI bus P.

The outputs of the grant decoder 604 (FIG. 10) are the D₋₋ GNT 6:0!signals which are provided to the grant storage registers 630 and to thegrant off logic 632. The grant off logic 632 also receives the GNT 6:0!signals as well as the PARK₋₋ CPU and PCI₋₋ IDLE signals. The grant offlogic 632 provides the GNT₋₋ OFF signal, which is used to guarantee onePCICLK of dead time between grants. Thus, if it is detected that twogrant lines are to transition at the same time, i.e., one grant line isbeing deasserted while another grant line is being asserted when the PCIbus P is idle, then the grant off logic 632 forces all grants inactivefor one PCICLK clock to comply and ensure the required dead time betweengrants during an idle PCI bus P.

The grant off logic 632 is shown in more detail in FIG. 12. The GNT₋₋OFF signal is produced as the output of a two-input AND gate 634, oneinput which is the PCI₋₋ IDLE signal. The other input to the AND gate634 is the output of an eight-input OR gate 640. The first input to theOR gate 640 is provided by the output of a three-input AND gate 642A,which receives at its inputs the GNT 0! signal, the PARK₋₋ CPU signaland a signal indicating that the D₋₋ GNT 6:1! signals are not equal to0b000000. The other inputs of the OR gate 640 are connected to theoutputs of three-input AND gates 642B-H. The inputs to the AND gates642B-H receive, respectively, signals GNT 0!, |D₋₋ GNT 0!, and a signalindicating D₋₋ GNT 6:1! not equal 0b000000; signals GNT 1!, |D₋₋ GNT 1!and a signal indicating D₋₋ GNT 6:2,0! not equal 0b000000; signals GNT2!, |D₋₋ GNT 2! and a signal indicating D₋₋ GNT 6:3,1:0! not equal0b000000; signals GNT 3!, |D₋₋ GNT 3! and a signal indicating D₋₋ GNT6:4,2:0! not equal 0b000000; signals GNT 4!, |D GNT 4! and a signalindicating D₋₋ GNT 6:5,3:0! not equal 0b000000; signals GNT 5!, |D₋₋ GNT5! and a signal indicating D₋₋ GNT 6,4:0! not equal 0b000000; andsignals GNT 6!, |D₋₋ GNT 6! and a signal indicating D₋₋ GNT 5:0! notequal 0b000000. The GNT₋₋ OFF signal is provided to clear the grantregister 630 (FIG. 10) to force all grant lines inactive for one PCICLKclock when the bus ownership is changing while the PCI bus P is idle.

The grant registers 630 are shown in more detail in FIG. 11. The D₋₋ GNTX! signal, X equal 0-6, is provided to the 1 input of a multiplexor 653,which is selected by the signal SET₋₋ GNT. The output of the multiplexor653 is connected to the D input of a D-type flip flop 650, whosenon-inverting output provides the GNT X! signal. The GNT X! signal isfed back to the 0 input of the multiplexor 653. The clear input of theflip-flop 650 receives the output of a three-input OR gate 652, which atits inputs receive the PCI₋₋ RESET signal, the CLR₋₋ GNT signal and theoutput of an AND gate 651. The AND gate 651 receives signals GNT₋₋ OFFand SET₋₋ GNT. The SET₋₋ GNT signal is produced as the output of atwo-input AND gate 636 (FIG. 12), one input of which is the NEW₋₋ GNTsignal and the other of which is the output of a two-input NAND gate638. The inputs to the NAND gate 638 are the GNT 1! and REQ 1! signals,indicating that the ISA bus I is requesting and has been granted the PCIbus P. Thus, the SET₋₋ GNT signal is provided when a new grant is tooccur and the ISA bus I is not the current master or requesting the PCIbus P.

The CLR₋₋ GNT signal is provided as the output of a three-input OR gate654 (FIG. 12). The inputs to the OR gate 654 are the MNGNT₋₋ CLR andGNT16₋₋ TO signals and the output of a two-input AND gate 656 whichreceives the EREQ₋₋ and |EGNT₋₋ signals. Thus, the EGNT₋₋ signal iscleared on the next rising edge PCICLK when the ISA request EREQ₋₋ isdeasserted while the signal EGNT₋₋ is asserted. The EGNT₋₋ signal isprovided as the output of a three-input NAND gate 658 whose inputs arethe GNT 1!, EGNT₋₋ EN and |LOCK₋₋ ACTV signals. The MNGNT₋₋ CLR andEGNT₋₋ EN signals are provided by the logic shown in FIG. 8. Thus, theCLR₋₋ GNT signal is used to clear the grant register 650 when a busmaster has been idle on the PCI bus P for more than 16 PCICLK clocks,the ISA request EREQ₋₋ has gone away while the ISA grant signal EGNT₋₋is asserted, or the minimum grant timer 414 has expired while the busowner is still active on the PCI bus P.

The GNT x! signal is also provided to the D inputs of D-type flip-flops658 and 660 (FIG. 11). The non-inverted output of the flip-flop 658 isthe SGNT x! or synchronized grant signal, while the non-inverted outputof the flip-flop 660 produces the SNGNT x! signal because the flip-flop660 is clocked on the falling edge of the PCICLK signal. Thus theflip-flops 658 and 660 are included in the synchronized grant register662 of FIG. 10.

One signal utilized in the new grant state machine 412 was the GNTS₋₋MINTO signal, which is also generated in the synchronized grant register662 (FIG. 10). This signal is produced at the output of a D-typeflip-flop 676 (FIG. 11). The clear input to the flip-flop 676 isprovided by the output of the OR gate 652. The D input of the flip-flop676 is provided by the output of a multiplexor 681, whose 0 inputreceives the signal GNTS₋₋ MINTO and whose 1 input is connected to theoutput of a six-input OR gate 680. The inputs of the OR gate 680 receivethe D₋₋ GNT 6:2,0! signals. These are the particular bus masters whichhave minimum grant times.

Certain miscellaneous logic and signals have been discussed in thisdescription and the logic is as follows. The PARK₋₋ CPU signal isproduced by the non-inverting output of a D-type flip-flop 670 (FIG.12). The D-input of the flip-flop 670 is connected to the output of athree-input AND gate 672. The inputs to the AND gate 672 are the PCI₋₋IDLE signal, the |REQ₋₋ ACTV or inverted request active signal, and theIGNTS₋₋ NOCPU signal. The GNTS₋₋ NOCPU signal is produced at the outputof a six-input OR gate 674 which receives at its inputs the GNT 6:1!signals. Thus the PARK₋₋ CPU signal is active when the bus has beenidle, no requests are active and there are no grants to masters otherthan the CPU.

The REQ₋₋ ACTV or request active signal is produced at the output of aseven-input OR gate 682 (FIG. 13) which receives at its inputs the REQ6:0! signals. Similarly, the REQ₋₋ ALL signal is produced as the outputof a six-input OR gate 684 which receives at its inputs all of therequest signals except for REQ 1!, that is, the ISA request.

The final signal is the REARB signal, which is produced at thenon-inverting output of a D-type flip-flop 686, whose D input isconnected to the output of a three-input OR gate 688. One input to theOR gate 688 is the GNT16₋₋ TO signal, while the second input isconnected to the output of a two input AND gate 690. The FRAME signaland the |SFRAME signal or inverted, synchronized FRAME signal areprovided to the AND gate 690. The final input to the OR gate 688 isconnected to the output of a two-input AND gate 689, whose inputsreceive signals SEGNT and |SSEGNT. The SEGNT signal is the EGNT signalor ISA grant line synchronized to the PCICLK signal. The |SSEGNT signalis the inverted version of SEGNT delayed by one PCICLK clock. The REARBor rearbitration signal indicates that access to the PCI bus P must berearbitrated for the next cycle, as indicated by the assertion of thetimeout signal GNT16₋₋ TO, new assertion of the signal SEGNT, or newassertion of the FRAME signal.

Referring now to FIG. 14, the state diagram of the first retry statemachine 422 (FIG. 4) is shown. On the rising edge of the signal PCI₋₋RESET, the state machine begins in state A, which is the idle state. Instate A, if the state machine detects the assertion of the signal FRAMEwhile the signal MIN₋₋ GNT₋₋ TO is deasserted, it transitions to stateB. Otherwise, it remains in state A. Thus, in a bus cycle with theminimum grant timer 414 enabled, the first assertion of the signal FRAMEcauses the state machine 422 to transition out of its idle state. Thestate machine 422 remains in state B until it either detects a retrycycle or a data transfer cycle. A retry cycle is indicated by theassertion of the STOP signal with the signals TRDY and FRAME bothdeasserted. When a retry cycle is detected, the state machinetransitions from state B to state D.

If the PCI target is capable of completing the data phase of the currentcycle, it asserts the signal TRDY. Thus, when the IRDY and TRDY signalsare both asserted, that indicates that the bus master and target areready to perform a data transfer. As a result, the state machinetransitions from state B to state C. Otherwise, the state machineremains in state B.

Once in state C, the data transfer state, the state machine 422 remainsin state C until the minimum grant timer 414 times out, as indicated bythe assertion of the MIN₋₋ GNT₋₋ TO signal. When the signal MIN₋₋ GNT₋₋TO is detected asserted, the state machine transitions from state C backto state A.

In state D, the state machine asserts the signal CLR₋₋ MIN₋₋ GNT₋₋ TO toclear the minimum grant timer 414. As noted earlier, the CLR₋₋ MIN₋₋GNT₋₋ TO signal is provided to one input of the OR gate 510 (FIG. 9) todrive the signal MIN₋₋ GNT₋₋ TMR₋₋ RST. From state D, the state machinereturns to state A on the next rising edge of PCICLK. Thus, effectively,the minimum grant timer 414 is cleared to allow the current bus masterto immediately relinquish control of the bus when the target retries themaster in the first cycle, i.e., before TRDY is asserted.

Referring now to FIG. 15, a state machine in the SD arbiter 212 isshown. The state machine contains 4 states: PCI, REF, IDE and ISA. ThePCI state is the reset and default state, where PCI-to-ISA cycles areallowed to run. The REF state allows refresh cycles to be run on the ISAbus I by the refresh controller 215. In the IDE state, the EDMAcontroller 204 is allowed to run data cycles to the IDE interface. Asindicated above, data transfers with the IDE devices require use of theSD bus on the ISA bus I. In the ISA state, the DMA controller 216 isallowed run DMA cycles on the ISA bus. Further, in the ISA state, aselected ISA bus master is allowed to run cycles on the ISA bus Ithrough a cascaded DMA channel in the DMA controller 216.

Requests that come into the SD arbiter 212 are labeled REF₋₋ SD₋₋ REQ,IDE₋₋ SD₋₋ REQ, and ISA₋₋ SD₋₋ REQ. The request line REF₋₋ SD₋₋ REQ isprovided by the non-inverting output of a D-type flip-flop 700 (FIG.16A), which is clocked on the rising edge of PCICLK. The D input of theflip-flop 700 is a signal RHOLD, which indicates a refresh request fromthe ISA bus arbiter 218A. The request line ISA₋₋ SD₋₋ REQ is provided bythe non-inverting output of a D-type flip-flop 702, which is alsoclocked by PCICLK. The D input of the flip-flop 702 is connected to asignal DHOLD, which is a request for the ISA bus I from the ISA busarbiter 218A. The request line IDE₋₋ SD₋₋ REQ is provided by the EDMAcontroller 204 to indicate a request for the ISA bus I by the IDEdevices.

On assertion of the signal PCI₋₋ RESET, the state machine in the SDarbiter 212 enters state PCI. In state PCI, a signal PCI₋₋ SD₋₋ GNT isasserted high to indicate that the PCI bus P has access to the ISA busI. If the signal REF₋₋ SD₋₋ REQ is asserted, and a signal PCI₋₋ INACTIVEis also asserted, the state machine transitions from state PCI to stateREF. The signal PCI₋₋ INACTIVE indicates that the PCI slave 208 is notcurrently active, a PCI-to-ISA cycle is not posted, there is no cyclerunning on the ISA bus I, a posted write cycle is not in progress, andthe PCI-ISA bridge 130 is not locked by a PCI master. In state REF, asignal REF₋₋ SD₋₋ GNT is asserted high. The signal REF₋₋ SD₋₋ GNT isprovided to the 1 input of a multiplexor 704, whose 0 input receives asignal RHLDA. The select input of the multiplexor receives a signalBCLKSM₋₋ NST₋₋ B, which indicates that the ISA bus clock BCLK hastransitioned high. The output of the multiplexor 704 is provided to theD input of a D-type flip-flop 706, which is clocked on the rising edgeof PCICLK. The output of the D flip-flop 706 drives the signal RHDLA,which is an acknowledge signal provided to the refresh controller 215(FIG. 2). In response to the signal RHDLA, the refresh controller 215takes control of the ISA bus I to run a refresh cycle. It is noted inthe preferred embodiment, while the refresh cycle is running on the ISAbus I, the PCI arbiter 212 is free to rearbitrate its requests and togrant the PCI bus P to any PCI master that wins the arbitration, asexplained above in FIGS. 3-14.

In state REF (FIG. 15), the SD arbiter state machine transitions back tostate PCI if the signal REF₋₋ SD₋₋ REQ is deasserted, indicating thatthe refresh operation has completed. In the transition back to statePCI, an output signal IDE₋₋ MASK is equated to IDE₋₋ MASK and ISA₋₋ SD₋₋REQ. The signal IDE₋₋ MASK is used to mask off the EDMA request lineIDE₋₋ SD₋₋ REQ. Thus, the IDE request is masked if the signal IDE₋₋ MASKis already set high and the ISA request signal is high. When thatcondition is true, the priority in the next arbitration cycle belongs tothe ISA bus I over the IDE request. The SD arbiter state machine staysin state REF when the REF₋₋ SD₋₋ REQ signal is asserted.

From state PCI, the state machine transitions to state IDE if thesignals PCI₋₋ INACTIVE, |REF₋₋ SD₋₋ REQ, IDE₋₋ SD₋₋ REQ, and |IDE₋₋ MASKare all true. This indicates that a REFRESH request has not beenasserted, the EDMA request line has been asserted, and the EDMA requestline has not been masked by the IDE₋₋ MASK signal. In state IDE, asignal IDE₋₋ SD₋₋ GNT is asserted high and provided to the EDMAcontroller 204 to indicate it has been granted the ISA bus I. In stateIDE, if the EDMA request line IDE₋₋ SD₋₋ REQ is deasserted by the EDMAstate machine 230, the state machine transitions from state IDE to statePCI. Otherwise the state machine remains in state IDE. As explainedabove, the EDMA controller 204 can be preempted off the SD bus under twoconditions: it can be bus master preempted by a request from the DMAcontroller 216, an ISA bus master, or the refresh controller 215; theEDMA controller 204 can be slave preempted by a PCI master desiring torun a cycle on the ISA bus I. In the transition from state IDE to statePCI, the signal IDE₋₋ MASK is equated with ISA₋₋ SD₋₋ REQ and |BLK₋₋MASK. The signal BLK₋₋ MASK is asserted high by the EDMA controller 204when it is slave preempted by a PCI master needing to run a cycle on theISA bus I. Assertion of the signal BLK₋₋ MASK blocks masking of theIDE₋₋ SD₋₋ REQ signal by maintaining the signal IDE₋₋ MASK low.

The bus master preemption signal BM₋₋ PRE is provided by an OR gate 710(FIG. 16A), which receives the signals ISA₋₋ SD₋₋ REQ and REF₋₋ SD₋₋REQ, indicating that a refresh or a DMA request is pending. This causesthe EDMA controller 204 to be bus master preempted, causing the IDErequest line to be masked when it gives up the SD bus. After the ISAcycle or refresh cycle is completed, the IDE request line is unmasked.

Finally, the SD arbiter state machine transitions from state PCI tostate ISA if the signals PCI₋₋ INACTIVE, |REF₋₋ SD₋₋ REQ, |EGNT₋₋, and|ISA₋₋ MASK are all true and either the signal IDE₋₋ SD₋₋ REQ isdeasserted or the signal IDE₋₋ MASK is asserted. This indicates that aREFRESH request has not been asserted, an IDE request has not beenasserted or the IDE mask bit is set high, the ISA bus I has been grantedaccess to the PCI bus P, and the EDMA controller 204 is not requestingthe PCI bus P or running an IDE cycle on the PCI bus P. The ISA₋₋ MASKsignal is provided by an OR gate 708, whose inputs receive signals IDE₋₋REQ and IDE₋₋ BUSY. The signal IDE₋₋ REQ indicates that the EDMAcontroller 204 is requesting control of the PCI bus P, and the signalIDE₋₋ BUSY indicates that an IDE cycle is running on the PCI bus P.

In state ISA, a signal ISA₋₋ SD₋₋ GNT is asserted high. This indicatesto the DMA controller 216 or ISA bus master that it now has control ofthe ISA bus I. The SD arbiter state machine remains in state ISA untilthe signal EGNT₋₋ is deasserted high. In that case, the state machinetransitions from state ISA back to state PCI, asserting the signal IDE₋₋MASK low.

The logic for generating the signal IDE₋₋ MASK is described as follows.The signal IDE₋₋ MASK is provided by a D-type flip-flop 720, which isclocked by the signal PCICLK and reset by RST₋₋. The D input of theflip-flop 720 is connected to the output of an AND gate 722, whoseinputs are connected to the outputs of a NAND gate 724 and a multiplexor726. The inputs of the NAND gate 724 receive the signal EGNT₋₋ and asignal indicating that the SD arbiter is in state ISA. The 0 input ofthe multiplexor 726 receives the signal IDE₋₋ MASK, and the 1 input isconnected to the output of an OR gate 730. The select input of themultiplexor 726 is connected to the output of an OR gate 728, whoseinputs are connected to the outputs of AND gates 736 and 738. The inputsof the AND gate 736 receive the inverted state of the signal REF₋₋ SD₋₋GNT and a signal indicating that the SD arbiter is in state REF, and theinputs of the AND gate 738 receive the inverted state of the signalIDE₋₋ SD₋₋ GNT and a signal indicating that the SD arbiter is in stateIDE. The inputs of the OR gate 730 are connected to the outputs of ANDgates 732 and 734. Two inputs of the AND gate 732 receive the signalsIDE₋₋ MASK and ISA₋₋ SD₋₋ REQ, and the third input is connected to theoutput of the AND gate 736. Two inputs of the AND gate 734 receive thesignal ISA₋₋ SD₋₋ REQ and the inverted state of the signal BLK₋₋ MASK,and the third input is connected to the output of the AND gate 738.

Thus, when the state machine transitions from state REF back to statePCI, the state of the signal IDE₋₋ MASK is determined by the output ofthe AND gate 732. When the state machine transitions from state IDE backto state PCI, the state of IDE₋₋ MASK is determined by the output of ANDgate 734. When the state machine transitions from state ISA back tostate PCI, the signal IDE₋₋ MASK is set low by the output of the NANDgate 724. If none of the above transitions are occurring, the signalIDE₋₋ MASK remains unchanged.

Referring now to FIGS. 16A and 16B, portions of the DMA controller 216are shown. As explained earlier, arbitration for the ISA bus I isdetermined by a two way "ping-pong" priority system. The DMA controller216 and the ISA bus masters are categorized as one requestor type, whilethe PCI masters, refresh controller 215 and EDMA controller 204 arecategorized as the other requester type. Once the DMA requestor type hasgained control of the ISA bus I, it is assigned the lowest priority forthe succeeding arbitration cycle. The first requester type is indicatedby a signal DMA₋₋ WINS, and the second requestor type is indicated by asignal PCI₋₋ WINS. The signal DMA₋₋ WINS is provided by an AND gate 800,whose inputs receive signals PCI₋₋ OWNER and DMAREQ. The signal PCI₋₋OWNER indicates that the DMA controller 216 or a 16-bit ISA master isnot currently the owner of the ISA bus I. As a result, the DMA requestortype should be given priority in the next arbitration cycle. The signalDMAREQ indicates that the DMA controller 216 has asserted a request. Thesignal DMA₋₋ WINS is provided to the input of an inverter 802, whoseoutput drives the signal PCI₋₋ WINS.

The signal DMA₋₋ WINS is also provided to the 1 input of a multiplexor804, whose output is connected to the D input of a D-type flip-flop 806.The flip-flop 806 is clocked on the rising edge of a signal BCLK₋₋,which is the inverted version of the ISA system clock BCLK. The outputof the flip-flop 806 provides a signal NEXT₋₋ DMA₋₋ WINS, which is fedback to the zero input of the multiplexor 804. The select input of themultiplexor 804 is connected to the output of a NAND gate 806, whoseinputs receive a signal RST₋₋ or system reset and a signal CHANGE. Theoutput of the NAND gate 806 provides a signal NEXT₋₋ CLK, which is anindication to determine the next winner.

The signal CHANGE is provided by the non-inverting output of a D-typeflip-flop 808, which is clocked by the BCLK signal. The flip-flop 808 ispreset high by the system reset signal RST₋₋. The D input of theflip-flop 808 is connected to the output of an AND gate 810, whose firstinput receives the inverted state of the signal CHANGE and whose secondinput is connected to the output of an OR gate 812. The inputs of the ORgate 812 are connected to the outputs of AND gate 814 and 816. The firstinput of the AND gate 814 receives a signal DMA₋₋ END or end of DMAcycle, and the second input receives the inverted state of a signalREF₋₋ END or refresh end cycle. The inputs of the AND gate 816 receivesignals DHOLD, which when asserted indicates a request from the DMAcontroller 216 or an ISA bus master, a signal DHLDA or the DMA grantacknowledge signal, and a signal DD₋₋ HLDA₋₋ which is the invertedversion of DHLDA delayed by one BCLK clock. Assertion of the signalCHANGE thus indicates the end of a DMA cycle or the grant of a new DMArequest. Assertion of the signal CHANGE causes the priority between thefirst requestor type and the second requester type to toggle.

The acknowledge signal DHLDA is provided by the PCI interface portion ofthe ISA bus controller 214. It is driven by the non-inverting output ofa D-type flip-flop 716, which is clocked by the PCICLK signal. The Dinput of the flip-flop 716 is provided by a multiplexor 715, whose 1input is connected to the output of an AND gate 712 and whose 0 input isconnected to the signal DHLDA. The select input of the multiplexor 715is connected to the output of an OR gate 714. The inputs of the OR gate714 receive signals BCLKSM₋₋ NST₋₋ C and BCLKSM₋₋ NST₋₋ D. The inputs ofthe AND gate 712 receive signals DHOLD, ISA₋₋ SD₋₋ GNT, the invertedstate of signal EGNT₋₋, and the inverted state of a signal S₋₋ EBM₋₋DONE. The signal S₋₋ EBM₋₋ DONE indicates that the PCI master logic 206is executing a posted write cycle on the PCI bus P. Thus, in response toa DHOLD request, assertion of the EGNT₋₋ signal, and assertion of thegrant signal ISA₋₋ SD₋₋ GNT by the SD arbiter 212, and the ISA buscontroller 214 is not currently completing a data transfer operation,the grant acknowledge signal DHLDA is asserted on the rising edge ofPCICLK while the BCLK signal is falling low or is already low, asindicated by the signals BCLKSM₋₋ NST₋₋ C and BCLKSM₋₋ NST₋₋ D,respectively. This ensures that the signal DHLDA satisfies various setupand hold time requirements with respect to the DMA controller 216.

The signal NEXT₋₋ DMA₋₋ WINS provided by the flip-flop 807 is providedto the input of a latch 818, one input of an AND gate 820 and one inputof an AND gate 822. The output of the latch is a signal DMA₋₋ OWNER, andthe latch is enabled by the signal CHANGE. Thus, when the state of thesignal CHANGE is asserted high, the state of the signal NEXT₋₋ DMA₋₋WINS is passed through the latch 818 to the signal DMA₋₋ OWNER. Thesignal DMA₋₋ OWNER indicates that the DMA controller 216 or an ISA busmaster has priority in the current arbitration cycle.

The other input of the AND gate 820 is connected to the inverted stateof a signal NEXT₋₋ CASCADE₋₋ MODE, which is provided by thenon-inverting output of a D-type flip-flop 824. The flip-flop 824 isclocked by the signal BCLK₋₋, and its D input is connected to the outputof a multiplexor 826. The 0 input of the multiplexor 826 is connected tothe signal NEXT₋₋ CASCADE₋₋ MODE and the 1 input is connected to asignal CASCADE₋₋ MODE. The signal CASCADE₋₋ MODE indicates that theselected channel of the DMA controller 214 can be programmed in cascademode. When a channel is programmed in cascade mode, that allows the ISAbus master assigned to that channel to request access to the ISA bus Ithrough the DMA controller 216. In the preferred embodiment, each of theseven channels in the DMA controller 216 can be programmed in cascademode. As each channel is programmed in cascade mode, arbitration for theISA bus I between the ISA bus masters are determined by the arbitrationscheme for 8237-compatible DMA channels. A 16-bit ISA bus masterrequests access to the ISA bus I by asserting its DRQ signal.

The output of the AND gate 820 drives the input of a latch 828, whoseoutput provides a signal DMA₋₋ ACT. The enable input of the latch 828 isconnected to the signal CHANGE. For compatibility reasons, the DMAcontroller 214 when programmed in the cascade mode or the ISA compatibletiming cannot be preempted, i.e., it can own the ISA bus I for as longas it wishes. However, if a particular channel in the DMA controller 214is not programmed to cascade mode or for compatible timing, then thatchannel can be preempted, causing the DMA controller 216 to relinquishcontrol of the ISA bus I if a bus request is indicated by a signalBUSREQ. Assertion of the signal DMA₋₋ ACT indicates that the particularchannel has not been programmed to cascade mode and therefore thechannel can be preempted. Further, assertion of the signal DMA₋₋ ACTindicates that the DMA controller 216 has control of the ISA bus I.

As noted, the signal NEXT₋₋ DMA₋₋ WINS is also provided to one input ofthe AND gate 822, whose other input receives the signal NEXT₋₋ CASCADE₋₋MODE. The output of the AND gate 822 is connected to the input of alatch 830, whose output provides a signal ISA₋₋ MASTER. The enable inputof the latch 830 is connected to the signal CHANGE. The signal ISA₋₋MASTER is provided to an inverter 832, which drives the signalEMSTR16₋₋. The signal EMSTR16₋₋ indicates that the ISA bus I has beengranted to a 16-bit ISA bus master, rather than the DMA controller 216.

The signal PCI₋₋ WINS is provided to the 1 input of a multiplexor 834,whose output is connected to the D input of a D-type flip-flop 836. Theflip-flop 836 is clocked by the signal BCLK₋₋ and provides a signalNEXT₋₋ PCI₋₋ WINS at its non-inverting output. The signal NEXT₋₋ PCI₋₋WINS is fed back to the 0 input of the multiplexor 834. The select inputof the multiplexor 834 receives the signal NEXT₋₋ CLK. The signal NEXT₋₋PCI₋₋ WINS is provided to the input of a latch 838, whose outputprovides a signal PCI₋₋ OWNER indicating that a PCI bus master or therefresh controller 215 is the winner in the current arbitration cycle.The enable input of the latch 838 receives the signal CHANGE.

The following shows the logic necessary to preempt a DMA channel off theISA bus I. The BUSREQ signal is provided by a D-type flip-flop 840,which is clocked by the signal BCLK₋₋. The D input is connected to theoutput of an OR gate 842, which receives signals REQ₋₋ ALL or therequest signals from the PCI masters, the signal RHOLD or the refreshrequest, the signal DMAREQ, and a signal S₋₋ DRQ or the synchronizedEDMA request. Thus, a request by a PCI master, the refresh controller215, the DMA controller 214, or the EDMA controller 204 causes thesignal BUSREQ to be asserted high. The signal BUSREQ is provided to oneinput of a 4-input AND gate 844 (FIG. 16B), whose other inputs receivesignals DMA₋₋ ACT, EN₋₋ DMA₋₋ TIME₋₋ CHK, and the inverted state of thesignal DMA END. The signal EN₋₋ DMA₋₋ TIME₋₋ CHK is an enable bit thatis set high to enable DMA time out. The output of the AND gate 844 isprovided to the enable input of a timer 846, which is clocked by thesignal BCLK. The timer 846 is a 5-bit counter that drives a signalTIMEOUT32 high when it counts to the value 31. The inverted state of theAND gate 844 is connected to the clear input of the timer 846. Thus,when the timer 846 is not enabled, it is cleared to the value 0. Thesignal TIMEOUT32 is provided to one input of an AND gate 848, whoseother input receives a signal DMA₋₋ ACT. The output of the AND gate 848is connected to one input of an OR gate 850. The output of the OR gate850 drives a signal STOP₋₋ DMA, which causes the DMA controller 214 tobe forced off the ISA bus I. The signal STOP₋₋ DMA is provided to the Dinput of a D-type flip-flop 852, which is clocked by the signal BCLK.The non-inverting output of the flip-flop 852 is connected to the otherinput of the OR gate 850. The clear input of the flip-flop 852 isconnected to the signal DMA₋₋ END. Generally, the signal DMA₋₋ END isasserted high at the end of a DMA cycle by the DMA controller. However,assertion of the signal STOP₋₋ DMA also causes the DMA controller toassert the signal DMA₋₋ END high on the next rising edge of BCLK. As aresult, one BCLK cycle after assertion of the signal STOP₋₋ DMA, thesignal DMA-END is asserted high to clear the flip-flop 852.

The signal DHOLD indicating a request from the DMA controller 216 or anISA bus master is provided by an OR gate 854 (FIG. 16A). As noted above,the signal DHOLD is provided to the SD arbiter 212. One input of the ORgate 854 receives the inverted state of the signal PCI₋₋ OWNER, whilethe other input is connected to the output of a D-type flip-flop 856.Thus, the signal DHOLD is allowed to be asserted only if a PCI masterdoes not currently have priority. The flip-flop 856 is clocked by thesignal BCLK, and its D input is connected to the output of an AND gate858. The clear input of the flip-flop 856 is connected to the output ofan OR gate 860. The first input of the OR gate 860 receives the invertedstate of the signal RST₋₋ and the second input receives a signalCHANGE₋₋ SHORT, which is pulsed high for half a BCLK cycle when thesignal CHANGE is asserted high. One input of the AND gate 858 receivesthe signal RST₋₋ and the other input is connected to the output of an ORgate 862. The first input of the OR gate 862 is connected to the outputof the flip-flop 856, and the other input receives a signal SYNC₋₋ DRQor a synchronized DMA request. Thus, upon assertion of a DMA request,the signal DHOLD is asserted high on the next rising edge of BCLK if thesystem reset signal RST₋₋ is deasserted and the signal PCI₋₋ OWNER islow.

Effectively, the ping-pong or alternating priority scheme between thefirst requestor type and the second requester type forces the DMAcontroller 216 or 16-bit ISA bus master to give up control of the ISAbus I after it has control for one arbitration cycle. As a result, thisgives the SD arbiter 212 the opportunity to provide access to the ISAbus to a PCI bus master, the EDMA controller 204, or the refreshcontroller 215.

The refresh request or RHOLD signal is provided by a two-input AND gate864, which receives a signal REFCNT and the inverted state of a signalREF₋₋ END. The signal REFCNT indicates that the refresh queue is notempty, which indicates that a REFRESH is pending. In the preferredembodiment, a refresh cycle is needed approximately every 15microseconds. The signal REF₋₋ END indicates the completion of a refreshcycle. As discussed above, the signal RHOLD is provided to the SDarbiter 212 to request control of the ISA bus I for the refreshcontroller 215.

Referring now to FIG. 17, logic in the flush request block 418 is shown.As noted above, a problem may arise when the CPU posts a write to theCPU-to-PCI queue of the bridge chip 110 for a processor-to-PCI writecycle, or when the PCI bus is locked by the bridge chip 110,particularly when the bridge chip 110 is implemented with the Intel82434X bridge chip. With the Intel chip, any attempt by a PCI master toaccess main memory 214 while either of the above two conditions is truewill result in the bridge chip 110 asserting a retry to the PCI master.Thus, if the minimum grant timer 414 for the PCI master is enabled (thatis, not loaded with the value 0), then this will result in the PCImaster not transferring any data for the remainder of the minimum granttime. This adversely affects bus performance. To overcome this problem,the flush request logic 418 asserts the FLUSHREQ₋₋ signal while one ofthe REQ 6:2! signals is asserted.

The signal FLUSHREQ₋₋ is provided by a NOR gate 900, whose first inputreceives a signal EREQ₋₋ FLUSHREQ and whose second input is connected tothe output of an OR gate 902. The signal EREQ₋₋ FLUSHREQ is assertedhigh when a state machine EREQ₋₋ SM (FIG. 18) in the PCI master logic206 detects a request from the ISA bus I. The five inputs to the OR gate902 are connected to signals FLUSH₋₋ REQ 6:2!. The signal FLUSH₋₋ REQX!, X equal to 2-6, is provided by a D flip-flop 904. The D flip-flop904 is clocked by PCICLK, and its clear input is connected to PCI₋₋RESET. The D input of the flip-flop 904 is connected to the output of atwo-input OR gate 906, whose first input is connected to the output ofan AND gate 908 and whose second input is connected to the output of athree-input NAND gate 910. The inputs of the AND gate 908 receivesignals REQ X! and MIN₋₋ TMR₋₋ EN X!. The signal REQ X! is provided bythe request mask logic 400. The signal MIN₋₋ TMR₋₋ EN X! is provided bya two-input AND gate 912, whose first input receives a signal ARB₋₋FLUSHREQ₋₋ EN which enables the assertion of the signal ARB₋₋ FLUSHREQ.The second input of the AND gate 912 is provided by a signal indicatingthat the minimum grant timer 414 for the selected PCI master is notloaded with the value zero.

The inputs of the AND gate 910 receive signals |SREQ X!, |GNT X! andMIN₋₋ GNT₋₋ TO. Thus, once a PCI master other than the bridge chip 110obtains control of the PCI bus P, the signal FLUSHREQ₋₋ remains asserteduntil the bus-master deasserts its request line. This prevents thebridge chip 110 from posting cycles to its write posting queue while aPCI master is accessing main memory 114.

As noted earlier in FIG. 14, further optimization is provided byclearing the minimum grant timer 414 when a retry occurs in the firstcycle of a transaction. This allows another PCI master to gain control.However, the flush request signal FLUSHREQ₋₋ is maintained asserteduntil the first PCI master is again able to get back onto the PCI bus Pin the next arbitration cycle to complete its transaction.

Referring now to FIG. 18, the state machine EREQ₋₋ SM is shown. Onsystem reset RST₋₋, the state machine enters state A. On the assertionof a signal SDHOLD or synchronized version of the DHOLD signal, and theassertion of a signal MEMACK₋₋ for acknowledging the receipt of thesignal FLUSHREQ₋₋, the state machine transitions to state B. Thus, uponreceipt of a request from the DMA controller 216, the state machinechecks the state of MEMACK₋₋ to ensure that another PCI master does notalready have the signal FLUSHREQ₋₋ asserted. In the transition to stateB, the signal EREQ₋₋ FLUSHREQ is set high. It is noted that once thesignal EREQ₋₋ FLUSHREQ is set to a particular value, its state remainsunchanged until noted otherwise.

The state machine remains in state B until the signal MEMACK₋₋ isasserted low to acknowledge the FLUSHREQ₋₋ signal and a signal EREQ₋₋ ENis set high. The signal EREQ₋₋ EN indicates that the EDMA controller 216is off the SD bus, that all the EDMA data buffers are flushed and thatthe EDMA controller 204 does not have the signal BLK₋₋ MASK asserted. Asa result, sufficient time would be allowed for an ISA bus master to gaincontrol of the ISA bus I.

Thus, if signals |MEMACK₋₋ and EREQ₋₋ EN are true, the state machinetransitions to state C and asserts the signal EREQ₋₋ low to indicate tothe PCI arbiter 210 a request from the ISA bus I. The signal EREQ₋₋ onceset low stays low until otherwise noted. The state machine remains instate C until the grant signal EGNT₋₋ is asserted low by the PCI arbiter210, which causes a transition to state D. Assertion of the signalEGNT₋₋ causes the acknowledge signal DHLDA to be asserted by the ISA buscontroller 214, at which time the state machine transitions to state E.The state machine remains in state E until the signal DHLDA is negatedlow. If both signals EBM₋₋ REQ and EBM₋₋ BUSY are deasserted low, thestate machine transitions from state E back to the idle state A. Thesignal EBM₋₋ BUSY indicates that the PCI master logic 206 in the PCI-ISAbridge 130 is busy performing a transfer on the PCI bus P. The signalEBM₋₋ REQ is provided by the ISA bus master 214 and indicates that anISA cycle is in progress and has not completed. In the transition fromstate E to state A, the signal EREQ₋₋ FLUSHREQ is deasserted low and thesignal EREQ₋₋ is deasserted high. If either of signals EBM₋₋ REQ OREBM₋₋ BUSY is asserted high, the state machine transitions from state Eto state F, where it remains until both signals EBM₋₋ REQ and EBM₋₋ BUSYare deasserted low. In the transition from state F to state A, thesignals EREQ₋₋ FLUSHREQ and EREQ₋₋ are deasserted.

Thus, an improved arbitration scheme has been described that includesmultiple arbiters for arbitrating access to a PCI bus and an ISA bus.The PCI arbiter controls access to the PCI bus by various bus masters,including the CPU/main memory subsystem, various other PCI bus masters,the EDMA controller, and an 8237-compatible DMA controller. The PCIarbiter utilizes a modified LRU arbitration scheme. The EDMA controllercontrols data transfers between IDE devices and main memory. An SDarbiter exists to arbitrate access to the data portion (SD) of the ISAbus. The various devices that may request the SD bus include the EDMAcontroller, a PCI master in a PCI-to-ISA operation, the DMA controller,an ISA bus master, and the refresh controller. The SD arbiter generallyassigns the highest priority to the PCI bus, followed by the refreshcontroller, EDMA controller, and DMA controller or ISA bus masters,noting that signals ISA₋₋ MASK and IDE₋₋ MASK may modify priority undercertain cases. The DMA controller includes an arbiter for arbitratingbetween its channels. The DMA arbiter further includes logic to ensurethat the DMA controller or ISA bus master relinquishes control of theISA bus after one arbitration cycle.

As noted above, the PCI master logic 206 (FIG. 2) includes twofour-double-word buffers 209A and 209B in the path from the IDE devicesto main memory 114. Referring now to FIG. 19, the first set offour-double-word buffers 209A comprise latches 1302A-H. The inputs tothe latches 1302A-H are connected to the 16-bit SD bus or SD 15:0!. Theenable inputs of the latches 1302A-H are connected, respectively, towrite latch enable signals IDE₋₋ WR₋₋ LE 0:7!. The outputs of thelatches 1302A-H provide, respectively, LWRDB0 15:0!, LWRDB0 31:16!,LWRDB1 15:0!, LWRDB1 31:16!, LWRDB2 15:0!, LWRDB2 31:16!, LWRDB3 15:0!,and LWRDB3 31:16!. Thus, when a latch enable signal IDE₋₋ WR₋₋ LE X! isasserted high, the state at the input of the corresponding latch will bepassed through to the output of the latch.

The second set of four-double-word buffers 209B comprise four 32-bitregisters 1304A-D, which are all clocked on the rising edge of anaddress/data register clock or a signal WRADR provided by the PCI masterlogic 206 to latch in PCI address and data at appropriate times duringPCI cycles. In this case, the WRADR signal is used to clock in the datafrom the first stage buffers 1302A-H into the second stage buffers1304A-D during the data phase of the PCI master logic 206. The inputs ofthe registers 1304A-D are connected, respectively, to signals LWRDB031:0!, LWRDB1 31:0!, LWRDB2 31:0!, and LWRDB3 31:0!, and their outputsprovide signals RWRDB0 31:0!, RWRDB1 31:0!, RWRDB2 31:0!, and RWRDB331:0!, which are in turn provided to the 0, 1, 2 and 3 inputs of amultiplexor 1306, respectively. The output of the multiplexor 1306provides signals IDE₋₋ WRDB 31:0!, which are provided to an input of amultiplexor 1308. The multiplexor 1308 drives signals WRIRDB 31:0!,which drive data to the PCI data bus PCIAD 31:0! during the data portionof a PCI cycle. The other inputs of the multiplexor 1308 are the writedata in an ISA-to-PCI (including DMA) transaction, read data in aPCI-to-ISA transaction, data from internal registers in the PCI-ISAbridge 130, and read data during I/O cycles.

The S1 and S0 select inputs of the multiplexor 1306 receive signalsGW41DT₋₋ MUX 1:0!, respectively. The signals GW41DT₋₋ MUX 1:0! areprovided by the PCI master logic 206 to route the appropriate doubleword of data onto the PCI data bus PCIAD during the data transfer phaseof a PCI write cycle.

As noted above, there is only one four-double-word buffer 211 for diskwrites (EDMA reads). It is noted that the buffers 211 are also used forPCI-to-ISA transfers controlled by the DMA controller 216 or ISA busmasters. As shown in FIG. 20, the read buffer comprises four 32-bitregisters 1350A-D, all clocked on the rising edge of PCICLK. The inputsof the registers 1350A-D are connected to the outputs of multiplexors1352A-D. The select inputs of the multiplexors 1352A-D receive,respectively, signals RD₋₋ DW 0:3!. The 0 inputs of the multiplexors1352A-D are connected to the PCI address/data bus PCIAD 31:0!. The 1inputs of the multiplexors 1352A-D are connected to the outputs ofregisters 1350A-D, respectively, which are referred to as signals RRDDB031:0!, RRDDB1 31:0!, RRDDB2 31:0!, and RRDDB3 31:0!. The PCI masterlogic 206 drives the RD₋₋ DW X! signal low to indicate when anappropriate double word of data is valid on the PCI data bus PCIAD fortransfer into the read buffer 211.

The outputs of the registers 1350A-D are provided to the 0, 1, 2 and 3inputs of a multiplexor 1354, whose output provides signals BMRDT 31:0!.The signals BMRDT 31:0! are multiplexed onto the 16-bit SD bus for datatransfer to the selected IDE device. The S1 and S0 select inputs of themultiplexor 1354 receive signals RD₋₋ SEL 1:0!.

The signals RD₋₋ SEL 1:0! are provided by a 2-bit register 1356, whichis clocked by PCICLK. The input of the register 1356 is connected to theoutput of a 4-to-2 multiplexor 1358, whose 0 input receives the signalsRD₋₋ SEL 1:0! and whose 1 input is connected to the output of a register1360. The select input of the multiplexor 1358 is connected to theoutput of an OR gate 1362, whose first input receives the inverted stateof the signal IDE₋₋ SD₋₋ GNT and whose second input is connected to theoutput of a two-input AND gate 1364. The inputs of the AND gate 1364receive signals IDE₋₋ SD₋₋ GNT and D₋₋ NEXT₋₋ RD. As described earlierin FIG. 15, the signal IDE₋₋ SD₋₋ GNT indicates that the SD bus has beengranted to the EDMA controller 204. The IDE next read signal D₋₋ NEXT₋₋RD is provided by the IDE state machine 252 and indicates when the nextaddress is available to adjust the data multiplexed select signals RD₋₋SEL 1:0! and to indicate a buffer HIT₋₋ MISS₋₋.

The register 1360 is clocked by the rising edge of a signal WR₋₋ LTA,and its D input receives address signals I₋₋ ADDR 3:2!, which are bits 3and 2 of the IDE address. The signal WR₋₋ LTA is provided by the outputof a three-input OR gate 1366, whose inputs receive an ISA bus master orDMA address latch signal ISA₋₋ WR₋₋ LTA, an IDE address strobe D₋₋ AD₋₋STB, and an IDE first read signal D₋₋ FIRST₋₋ RD.

As explained above, the timing for an EDMA transfer is completely userprogrammable via PCI configuration I/O registers. There exists one setof configuration timing registers for the primary IDE devices 230 andanother set of configuration timing registers for the secondary IDEdevices 232. In addition to the total cycle time of an IDE transferbeing programmable, the active period and inactive period within eachEDMA cycle are also programmable. Thus, referring now to FIG. 21, thereis a primary active timing register 1600, a secondary active timingregister 1602, a primary inactive timing register 1604, and a secondaryinactive timing register 1606. The registers 1600-1606 are 5-bit deviceshaving their inputs connected to the outputs of multiplexors 1608, 1610,1612 and 1614, respectively. The 0 inputs of the multiplexors 1608,1610, 1612 and 1614 are connected to the outputs of the registers 1600,1602, 1604 and 1606, respectively. The 1 inputs of the multiplexors 1608and 1610 receive signals PCI₋₋ WRDT 4:0! which are latched versions ofthe PCI write data from the PCI data bus PCIAD. The 1 inputs ofmultiplexors 1612 and 1614 receive signals PCI₋₋ WRDT 12:8!. The selectinputs of the multiplexor 1608, 1610, 1612 and 1614 receive signalsTIMSEL0, TIMSEL1, TIMSEL2 and TIMESEL3, respectively. Signals TIMSEL0:3! are asserted during configuration write cycles to program theregisters 1600-1606 with the desired values to control the active andinactive periods of an EDMA cycle. Each of the registers 1600-1606 areclocked on the rising edge of the signal PCICLK, and are preset high bythe inverted PCI reset or PCI₋₋ RESET₋₋ signal.

The output of the register 1600 and the output of the register 1602 areconnected to the 1 and 0 inputs of a multiplexor 1624, respectively. Themultiplexor 1624 is selected by a primary/secondary signal PRI₋₋ SEC₋₋,which when set high indicates the selection of a primary master or slaveIDE device. The outputs of the registers 1604 and 1606 are connected tothe 1 and 0 inputs of a multiplexor 1628, respectively, which is alsoselected by the signal PRI₋₋ SEC₋₋. The outputs of the multiplexors 1624and 1628 are provided to the 1 and 0 inputs of a multiplexor 1630,respectively, which is selected by a signal ACT₋₋ INACT₋₋ to indicatethe selection of the active or inactive time period. When high, thesignal ACT₋₋ INACT₋₋ indicates the active period. The output of themultiplexer 1630 is connected to the 1 input of a multiplexor 1632,whose select input receives a signal LOAD₋₋ TIME. The output of themultiplexor 1632 provides signals M₋₋ CNT 4:0!, which are provided tothe input of a decrementer 1634. The output of the decrementer 1634,referred to as signals S₋₋ CNT 4:0!, are connected to one input of acomparator 1638 and the input of a 5-bit register 1636. The comparator1638 asserts a signal TIME₋₋ CNT₋₋ 0 high when the signal S₋₋ CNT 4:0!reaches the value 0. The register 1636 is clocked on the rising edge ofPCICLK and is cleared by the signal PCI₋₋ RESET₋₋. The output of theregister 1636 is fed back to the 0 input of the multiplexor 1632. Thus,the combination of the multiplexer 1632, the decrementer 1634, theregister 1636, and the comparator 1638 form an active/inactive timer1631, which begins counting from the value indicated in one of theprimary or secondary timing registers 1600, 1602, 1604 or 1606. Thesignal TIME₋₋ CNT₋₋ 0 when asserted indicates the end of either theactive or inactive portion of an EDMA transfer.

Thus, in accordance with the present invention, FIG. 21 shows the logicnecessary to enable the programmability of the timing of an EDMAtransfer.

Referring now to FIG. 22, the state diagram of the EDMA state machine250 in the EDMA controller 204 is shown. The output signals provided bythe EDMA state machine 250 include a "D₋₋ " prefix to indicateconnection to the D input of a corresponding D-type flip-flop clocked byPCICLK and cleared by the system reset RST₋₋. The output of theflip-flop is the signal name without the "D₋₋ " prefix. In the ensuingdiscussion, the output signals of the EDMA state machine 250 maintaintheir state until changed. The exceptions are signals D₋₋ DRQ₋₋ PRE andD₋₋ FIRST₋₋ RD, which are set low on each PCICLK signal unless otherwiseindicated.

On system reset, indicated by |RST₋₋, the EDMA state machine 250 entersstate IDLE. The state machine transitions out of state IDLE to state GNTif signals |IDE₋₋ SD₋₋ GNT, |IDE₋₋ BUSY, and D₋₋ IDE₋₋ IDLE are alltrue, the EREQ₋₋ signal is deasserted or high, and either a primary orsecondary IDE device is requesting service by the S₋₋ IDE₋₋ DRQ₋₋ P orS₋₋ IDE₋₋ DRQ₋₋ S signals. The signal IDE₋₋ SD₋₋ GNT, asserted by the SDarbiter 212 to indicate to the EDMA controller 204 that it has controlof the SD bus, is checked to ensure that it is deasserted to prevent thepossibility of having data from both the primary and secondary devicesin the write buffers 209A and 209B. The signal IDE₋₋ BUSY beingdeasserted low indicates that the PCI master logic 206 is not busy witha previous write cycle. If there are still posted data in the writebuffers, the signal IDE₋₋ BUSY will be asserted. The signal EREQ₋₋ ischecked to ensure that a request for the PCI bus P is not asserted byone of the ISA bus masters. The signal D₋₋ IDE₋₋ IDLE asserted indicatesthat the IDE state machine 252 is idle.

In the transition from state IDLE to state GNT, the signal D₋₋ IDE₋₋SD₋₋ REQ is asserted high and provided to the SD arbiter 212 to indicatea request for the SD bus. Further, the primary/secondary signal D₋₋PRI₋₋ SEC₋₋ and latched write/read signal D₋₋ IDE₋₋ LW₋₋ R are assignedvalues depending on the state of signals S₋₋ IDE₋₋ DRQ₋₋ P (synchronizedprimary IDE device request) or S₋₋ IDE₋₋ DRQ₋₋ S (synchronized secondaryIDE device request). If the signal S₋₋ IDE₋₋ DRQ₋₋ P is asserted, thenthe signal D₋₋ PRI₋₋ SEC₋₋ is set high to indicate data transferinvolving the primary IDE device. The signal D₋₋ IDE₋₋ LW₋₋ R₋₋ is setequal to the state of a signal PRI₋₋ EDMA₋₋ W₋₋ R₋₋ which is latchedduring a PCI I/O cycle to a primary IDE device and indicates if the IDErequest is a read (high) or a write (low). If the signal S₋₋ IDE₋₋ DRQ₋₋S is high, then the signal D₋₋ PRI₋₋ SEC₋₋ is set low to indicate thatthe transfer involves the secondary IDE device, and the signal D₋₋ IDE₋₋LW₋₋ R₋₋ is set equal to the state of SEL₋₋ EDMA₋₋ W₋₋ R.

The state machine remains in state GNT until the signal IDE₋₋ SD₋₋ GNTis asserted by the SD arbiter 212 to indicate that the EDMA controller204 has been granted control of the SD bus. There are six possibletransitions out of state GNT. In all transitions out of state GNT, thesignal D₋₋ BLK₋₋ MASK is set low. As indicated above, when the EDMAcontroller 204 is slave preempted by a PCI master, the EDMA controller204 asserts the signal BLK₋₋ MASK to prevent masking of the IDE requestin the next arbitration cycle. If the signal EREQ₋₋ is asserted low,indicating an active request for the PCI bus P from an ISA bus master,the state machine returns to state IDLE. In the transition, the signalD₋₋ IDE₋₋ SD₋₋ REQ is set low indicating that the EDMA controller 204 isno longer requesting the SD bus, and a signal D₋₋ IDE₋₋ VB₋₋ CLR or IDBread cycle valid bit clear is set high.

If the signal EREQ₋₋ is deasserted, the signal D₋₋ IDE₋₋ LW₋₋ R₋₋ is sethigh to indicate a write cycle, and the signal S₋₋ DRQ is assertedindicating a request from one of the IDE devices, then a data writetransfer from the IDE device to main memory 114 is ready to begin andthe state machine transitions to state WDAT. The signal S₋₋ DRQ isprovided by a multiplexor 1500 (FIG. 23B), whose 0 and 1 inputs receivethe signal S₋₋ IDE₋₋ DRQ₋₋ S and S₋₋ IDE₋₋ DRQ₋₋ P, respectively. Theselect input of the multiplexor 1500 receives the signal PRI₋₋ SEC₋₋. Ifthe signal S₋₋ DRQ is deasserted, indicating that the primary orsecondary IDE device is not yet ready to perform the write datatransfer, the state machine transitions from state GNT to state WDRQ. Inthe transition from state GNT to state WDAT, a signal D₋₋ XFER₋₋ EN isset high for indicating that data transfer is enabled and for triggeringthe IDE state machine 252. In the transition to state WDRQ, the signalD₋₋ XFER₋₋ EN set low. In the transition to either state WDRQ or WDAT,the signal D₋₋ IDE₋₋ WS is set low. The signal IDE₋₋ WS is provided tothe IDE state machine 252 to place it in a wait state while the writedata buffer 209B is being emptied.

In state GNT, if the signal IDE₋₋ LW₋₋ R₋₋ is set low indicating a readcycle, a signal HIT₋₋ MISS₋₋ is asserted high, and the signal S₋₋ DRQ isasserted high, then the state machine transitions to state RDAT. Thesignal HIT₋₋ MISS₋₋ is asserted high during a read cycle if the addressis in the same page (i.e. address bits 4-27 of the current IDE cycle arethe same as the previous cycle) and the data associated with the currentaddress is indicated as being valid. The read data is indicated as beingvalid if the appropriate one of the signals RD₋₋ DW 0:3! indicates thatvalid data has been latched into a corresponding one of the registers1350A-D. There is a bit corresponding to each double word of read datato indicate if the double word is valid. For an EDMA write cycle, thecycle is a hit if the address is in the same page and the write buffers209A-B are not full. The signal HIT₋₋ MISS₋₋ is provided by an OR gate1564 (FIG. 23B) In the transition from state GNT to state RDAT, thesignal D₋₋ XFER₋₋ EN is set high and the signal D₋₋ IDE₋₋ WS is set low.

The state machine transitions from state GNT to state RDRQ if the signalIDE₋₋ LW₋₋ R₋₋ is set low, the signal HIT₋₋ MISS₋₋ is asserted high, butthe signal S₋₋ DRQ is deasserted low. The transition to state RDRQindicates a read hit, but the request line IDE₋₋ DRQ₋₋ P or IDE₋₋ DRQ₋₋S from the primary or secondary IDE device has not been asserted. As aresult, the state machine remains in state RDRQ to wait for theassertion of S₋₋ DRQ. In the transition from state GNT to state RDRQ,signals XFER₋₋ EN and D₋₋ IDE₋₋ WS are set low.

Finally, the state machine transitions from state GNT to state RMIS if aread is indicated by the signal IDE₋₋ LW₋₋ R₋₋ but the signal HIT₋₋ MISSis set low indicating that either the current address is not in the samepage as the previous address or the data is not valid. In the transitionfrom state GNT to state RMIS, the signals D₋₋ XFER₋₋ EN and D₋₋ IDE₋₋ WSare set low, and a signal D₋₋ IDE₋₋ REQ or IDE PCI master request is sethigh to reread the data. The signal D₋₋ FIRST₋₋ RD is also set high. Thesignal D₋₋ FIRST₋₋ RD is provided to the OR gate 1366 (FIG. 20) to latchin read address bits I₋₋ ADDR 3:2!.

There are four possible transitions out of state WDRQ. If a signal BUF₋₋EMPTY is asserted indicating that the write buffers are empty and eithera signal BM₋₋ PRE or a signal DRQ₋₋ MASKED is asserted, then the statemachine transitions to state IDLE. The bus master preemption signal BM₋₋PRE is provided by an AND gate 1504 (FIG. 23A), which receives thesignal ISA₋₋ SD₋₋ REQ or ISA request and a signal REF₋₋ SD₋₋ REQ orrefresh SD request. The signal DRQ₋₋ MASKED is provided by a multiplexor1502, whose 0 and 1 inputs receive signals SEC₋₋ MASKED and PRI₋₋MASKED, respectively. The select input of the multiplexor 1502 receivesthe signal PRI₋₋ SEC₋₋. The signals PRI₋₋ MASKED and SEC₋₋ MASKED arecontrol register bits for indicating that I/O requests to the primaryand secondary IDE devices, respectively, are masked. The transition fromstate WDRQ to state IDLE indicates either that there is a bus masterpreemption or the IDE devices are masked, but the write data buffers209A and 209B are empty. As a result, the EDMA state machine 250 givesup control of the SD bus by setting the signal D₋₋ IDE₋₋ SD₋₋ REQ low.In the transition from state WDRQ to state IDLE, the signal D₋₋ IDE₋₋VB₋₋ CLR is set high to clear the read data valid bits.

If, however, the term (BUF₋₋ EMPTY & (BM₋₋ PRE ∥ DRQ₋₋ MASKED)) is nottrue and a slave preemption is indicated by assertion of a signal SLV₋₋PRE, the state machine transitions from state WDRQ to state NGNT. Asindicated, the EDMA controller 204 is slave preempted if a PCI masterdrives a cycle onto the PCI bus P with the PCI-ISA bridge 130 as thetarget, but the EDMA controller 204 is currently busy on the SD bus. Inthis case, the signal IDE₋₋ SD₋₋ REQ is set low to take the EDMAcontroller 204 off the SD bus. The signal BLK₋₋ MASK is set high andprovided to the SD arbiter 212 (FIG. 15) to prevent the IDE₋₋ SD₋₋ REQsignal from being masked in the next arbitration cycle.

The state machine transitions from state WDRQ to state WDAT if the term(BUF₋₋ EMPTY & (BM₋₋ PRE ∥ DRQ₋₋ MASKED)) is not true, the signal SLV₋₋PRE is deasserted low, and the signal S₋₋ DRQ is asserted high. Asnoted, state WDRQ is the state in which the EDMA state machine 250 iswaiting for the assertion of the signal S₋₋ DRQ which indicates eitherthe primary or secondary IDE device is ready to perform a data transfer.If the signal S₋₋ DRQ is detected asserted, the state machinetransitions to state WDAT, setting the signal D₋₋ XFER₋₋ EN high.

If the write data buffers 209A and 209B are not empty as indicated bythe signal BUF₋₋ EMPTY being deasserted low, and either the signal BM₋₋PRE or the signal DRQ₋₋ MASKED is asserted high, then the state machinetransitions to state WMIS. In the transition, the PCI bus master requestor D₋₋ IDE₋₋ REQ signal is set high. A signal DRQ₋₋ PRE is also set highto indicate that the IDE request has been bus master preempted. Thesignal DRQ₋₋ PRE interrupts the write latch state machine 254, causingit to transition to a state where the write latch enable signals IDE₋₋WR₋₋ LE 7:0! are set to the value 0x00. This tells the write latch statemachine 254 that a buffer boundary is not going to be reached and sothat the write latch state machine 254 should go to the LTCH state tokeep all of the write latches closed until the first level write datahas been transferred to the second level write posting buffer throughthe assertion of WRADR.

Once in state WDAT, there are three possible transitions out of thestate. The state machine remains in state WDAT if a signal D₋₋ DATA₋₋VLD is deasserted, which is provided by the IDE state machine 252 andindicates if a data transfer to or from the IDE device has justoccurred. If such a transfer has occurred, then the IDE state machine252 asserts the signal D₋₋ DATA₋₋ VLD in the last cycle of the activeperiod to indicate that data will be valid on the next rising edge ofPCICLK. In addition, if either a signal PSID₋₋ TC is asserted high orthe signal HIT₋₋ MISS₋₋ is deasserted low, then the state machinetransitions from state WDAT to state WMIS. The signal PSID₋₋ TC isprovided by a multiplexor 1506 (FIG. 23A), whose 0 and 1 inputs receivesignals SEC₋₋ CNT₋₋ 0 and PRI₋₋ CNT₋₋ 0. The select input of themultiplexor 1506 receives the primary/secondary signal PRI₋₋ SEC₋₋. Thesignals PRI₋₋ CNT₋₋ 0 and SEC₋₋ CNT₋₋ 0 indicate that a primary orsecondary byte counter, respectively, for keeping track of the number ofbytes to be transferred, has decremented down to the value 0xFFFF. Thus,if the signal PSID₋₋ TC or primary or secondary IDE terminal count isasserted, then the transfer is completed. In the transition, the PCImaster request or D₋₋ IDE₋₋ REQ signal is asserted. Additionally, if thesignal PSID₋₋ TC is asserted, then the signal D₋₋ DRQ₋₋ PRE is assertedto cause the write latch state machine 254 to flush the write buffersand to drive the write latch enable signals IDE₋₋ WR₋₋ LE 7:0! to thevalue 0x00.

Further, in the transition from state WDAT (FIG. 22) to state WMIS, ifany of the signals PSID₋₋ TC, BM₋₋ PRE, or |S₋₋ DRQ are true, then thesignal D₋₋ XFER₋₋ EN is set low to stop the IDE state machine 252. Thisalso causes the acknowledge line IDE₋₋ DAK₋₋ P₋₋ or IDE₋₋ DAK₋₋ S₋₋ tobe deasserted by the state machine 254. Thus, a bus master preemption inconjunction with the completion of a data transfer or with a miss of thewrite buffer will cause the IDE state machine 252 to go back to its idlestate instead of placing it in a wait state, as is usually the case.

However, if none of the signals PSID₋₋ TC, BM₋₋ PRE, and |S₋₋ DRQ aretrue, that is, the signal |HIT₋₋ MISS₋₋ is true and a write buffer misshas occurred, then the IDE state machine 252 is placed in a wait stateuntil the write buffers 209A and 209B free up. This is done by settingthe signal D₋₋ IDE₋₋ WS high.

If the signal D₋₋ DATA₋₋ VLD is asserted, but the term (PSID₋₋ TC ∥|HIT₋₋ MISS₋₋) is not true and the slave preemption or SLV₋₋ PRE signalis true, then the state machine transitions from state WDAT to stateNGNT. In the transition, the signal D₋₋ BLK₋₋ MASK is set high to blockthe masking of the IDE SD request, the signal D₋₋ XFER₋₋ EN is set lowto stop the IDE state machine 252, and the signal D₋₋ IDE₋₋ SD₋₋ REQ isset low which causes the EDMA controller 204 to give up control of theSD bus.

Finally, if the signal D₋₋ DAT₋₋ VLD is asserted, the term (PSID₋₋ TC ∥|HIT₋₋ MISS₋₋) is not true, a slave preemption has not occurred, and thesignal |S₋₋ DRQ is deasserted indicating that the IDE request lines havebeen deasserted, then the EDMA state machine 250 transitions from stateWDAT to state WDRQ to wait for reassertion of the signal S₋₋ DRQ. In thetransition, the signal D₋₋ XFER₋₋ EN is set low to stop the IDE statemachine 252.

As discussed above, the EDMA state machine 250 transitions to state WMISas a result of a bus master preemption, if the IDE DRQ lines are masked,or a write buffer miss occurred. There are three possible transitionsout of state WMIS. The EDMA state machine 250 remains in state WMIS ifthe signal WRADR or write address/data register clock is deasserted low.

While the state machine is in state WMIS, if the slave preemption SLV₋₋PRE or the bus master preemption signal BM₋₋ PRE is asserted, then thesignal D₋₋ IDE₋₋ SD₋₋ REQ remains low to keep the EDMA controller 204off the SD bus, the signal D₋₋ XFER₋₋ EN remains low to keep the IDEstate machine 252 in the IDLE state, and the signal D₋₋ IDE₋₋ WS is setlow to prevent the IDE state machine 252 from entering its wait state.

The EDMA state machine 250 transitions from state WMIS to state IDLE ifthe signal WRADR is asserted and one of the following conditions istrue: the signal PSID₋₋ TC is asserted to indicate completion of a datatransfer cycle; the bus master preemption signal BM₋₋ PRE is asserted;or the IDE SD request line IDE₋₋ SD₋₋ REQ is deasserted and the slavepreemption signal SLV₋₋ PRE is asserted indicating that slave preemptionhas occurred requiring the EDMA controller 204 to rearbitrate for the SDbus. In the transition to state IDLE, the signals D₋₋ IDE₋₋ REQ, D₋₋XFER₋₋ EN, D₋₋ IDE₋₋ WS, and D₋₋ IDE₋₋ SD₋₋ REQ are set low. The signalD₋₋ IDE₋₋ VB₋₋ CLR is set high to clear the read data valid bits.

If the term (PSID₋₋ TC ∥ BM₋₋ PRE ∥ (|IDE₋₋ SD₋₋ REQ & SLV₋₋ PRE)) isnot true, but the signal S₋₋ DRQ is asserted, then that indicates thatthe data buffers have been emptied and the IDE devices are ready totransfer some more data. As a result, the EDMA state machine 250transitions to state WDAT, setting the signals D₋₋ IDE₋₋ REQ and D₋₋IDE₋₋ WS low.

If neither of the above two conditions is true, then that indicates thatthe write buffers have been emptied, but the request signal S₋₋ DRQremains deasserted. In this case, the EDMA state machine 250 transitionsfrom state WMIS to state WDRQ to wait for the assertion of the signalS₋₋ DRQ. In the transition, the signals D₋₋ IDE₋₋ REQ, D₋₋ XFER₋₋ EN,and D₋₋ IDE₋₋ WS are set low.

Proceeding now to the read portion of the EDMA state machine 250, thevalid bit clear signal D₋₋ IDE₋₋ VB₋₋ CLR is set low in state RDRQ.State RDRQ is the state in which the state machine waits for theassertion of the signal S₋₋ DRQ. If the slave preemption signal SLV₋₋PRE is asserted, then the EDMA state machine 250 transitions from stateRDRQ to state NGNT. In the transition, the signal D₋₋ BLK₋₋ MASK is sethigh to block the masking of the IDE SD request signal IDE₋₋ SD₋₋ REQ,and the signals D₋₋ XFER₋₋ EN and D₋₋ IDE₋₋ WS are set low to stop theIDE state machine 252. In addition, the D₋₋ IDE₋₋ SD₋₋ REQ is set low toallow the SD arbiter 212 to give control of the SD bus to anothermaster.

The EDMA state machine 250 transitions from state RDRQ to state IDLE ifthe slave preemption SLV₋₋ PRE is deasserted and one of the followingtwo conditions is true: the signal BM₋₋ PRE is asserted and the signalS₋₋ DRQ is deasserted indicating a bus master preemption while therequest line S₋₋ DRQ is deasserted; or, the signal DRQ₋₋ MASKED isasserted for masking the primary and secondary request lines IDE₋₋ DRQ₋₋P and IDE₋₋ DRQ₋₋ S. In the transition back to state IDLE, the signalsD₋₋ XFER₋₋ EN, D₋₋ IDE₋₋ SD₋₋ REQ, and D₋₋ IDE₋₋ WS are set low. Thesignal D₋₋ IDE₋₋ VB₋₋ CLR is set high.

If the conditions triggering the transitions from state RDRQ to eitherstate NGNT or state IDLE are not true, and the signal S₋₋ DRQ isasserted, then the state machine transitions from state RDRQ to stateRDAT. This indicates that the PCI master logic 206 is no longer busy,allowing more data to be transferred to the read buffer 211. In thetransition to RDAT, the signal D₋₋ XFER₋₋ EN is set high to enable theIDE state machine 252, and the signal D₋₋ IDE₋₋ WS is set low to allowthe IDE state machine 252 to come out of its wait state.

There are four possible transitions out of state RDAT. The EDMA statemachine 250 remains in state RDAT until the signal D₋₋ DATA₋₋ VLD isasserted. The IDE state machine 252 asserts the signal D₋₋ DATA₋₋ VLD atthe end of the active period if a data transfer has occurred. If thesignal D₋₋ DATA₋₋ VLD is asserted, then the state machine transitionsfrom state RDAT back to state IDLE if one of the following twoconditions occur: the signal PSID₋₋ TC is asserted indicating completionof the data transfer; or the signal BM₋₋ PRE is asserted and the signalHIT₋₋ MISS₋₋ is deasserted indicating a bus master preemption and readbuffer miss. In the transition to state IDLE, the signal D₋₋ XFER₋₋ ENis set low, and the signal D₋₋ IDE₋₋ SD₋₋ REQ is set low to allow theEDMA controller 204 to get off the SD bus. In addition, the signal D₋₋IDE₋₋ VB₋₋ CLR is set high to clear the read data valid bits.

If the signal D₋₋ DATA₋₋ VLD is asserted but the term (PSID₋₋ TC ∥ (BM₋₋PRE & |HIT₋₋ MISS₋₋)) is not true, and the slave preemption signal SLV₋₋PRE is asserted, then the EDMA state machine 250 transitions from stateRDAT to state NGNT. In the transition to state NGNT, the signal D₋₋BLK₋₋ MASK is set high, and the signals D₋₋ XFER₋₋ EN and D₋₋ IDE₋₋ SD₋₋REQ are set low.

If the signal D₋₋ DATA₋₋ VLD is asserted, the term (PSID₋₋ TC ∥ (BM₋₋PRE & |HIT₋₋ MISS₋₋)) is not true, the signal SLV₋₋ PRE is deasserted,and the signal HIT₋₋ MISS₋₋ is deasserted low, then that indicates aread buffer miss which requires more read data to be obtained by the PCImaster logic 206 from main memory 114. This causes the EDMA statemachine 250 to transition from state RDAT to state RMIS. On the readbuffer miss, the valid bit clear signal D₋₋ IDE₋₋ VB₋₋ CLR is set highto clear the valid bits. The PCI master request signal D₋₋ IDE₋₋ REQ isset high to request control of the PCI bus P. In the transition, if thesignal S₋₋ DRQ is asserted, that indicates the IDE device is ready toaccept more data, i.e., a multi-data transfer transaction is occurring.As a result, the IDE acknowledge signal IDE₋₋ DAK₋₋ P₋₋ or IDE₋₋ DAK₋₋S₋₋ is maintained asserted by placing the IDE state machine 252 in itswait state. This is accomplished by setting the signal D₋₋ IDE₋₋ WShigh.

If, however, a read buffer miss has occurred but the signal S₋₋ DRQ isdeasserted, then a single data transfer is probably indicated. As aresult, in the transition from state RDAT to state RMIS, the signal D₋₋XFER₋₋ EN is set low to allow the IDE state machine 252 to transitionback to its IDLE state and deassertion of the corresponding IDE₋₋ DAK₋₋P₋₋ or IDE₋₋ DAK₋₋ S₋₋ signals.

If the signal D₋₋ DATA₋₋ VLD is asserted, but the terms (PSID₋₋ TC ∥(BM₋₋ PRE & |HIT₋₋ MISS₋₋)), SLV₋₋ PRE, and |HIT₋₋ MISS₋₋ are all nottrue, and if the signal S₋₋ DRQ is deasserted, then that indicates theIDE request line IDE₋₋ DRQ₋₋ P or IDE₋₋ DRQ₋₋ S has been deasserted. Inresponse, the EDMA state machine 250 transitions from state RDAT tostate RDRQ, setting the signal D₋₋ XFER₋₋ EN low to place the IDE statemachine 252 back into the IDLE state.

The EDMA state machine 250 remains in state RMIS if the signal WRADRremains deasserted. In state RMIS, the signal D₋₋ IDE₋₋ VB₋₋ CLR is setlow. If the signal WRADR is asserted, then the state machine transitionsto state RBSY to wait for the PCI master logic 206 to receive the readdata from the main memory 114. In the transition to state RBSY, the PCImaster request signal D₋₋ IDE₋₋ REQ is set low.

In state RBSY, the PCI master logic 206 is either waiting to obtain thePCI bus P or is actually performing the PCI cycle to retrieve the readdata from main memory 114. The PCI master logic 206 indicates this byasserting a signal IDE₋₋ BUSY. Thus, if the signal IDE₋₋ BUSY isasserted, the EDMA state machine 250 remains in state RBSY. If thesignal IDE₋₋ BUSY is deasserted, and the signal IDE₋₋ SD₋₋ REQ isasserted indicating a request for the SD bus and the slave preemptionsignal SLV₋₋ PRE is deasserted, and further the signal S₋₋ DRQ isasserted, the EDMA state machine 250 transitions from state RBSY tostate RDAT. In the transition, the signal D₋₋ IDE₋₋ WS is set low totake the IDE state machine 252 out of its wait state, and the signal D₋₋XFER₋₋ EN is set high to enable the IDE state machine 252 to perform thedata transfer.

However, if the term (IDE₋₋ SD₋₋ REQ & |SLV₋₋ PRE) is true but thesignal S₋₋ DRQ is deasserted, then that indicates the bus master logic206 has received the data, but the IDE device has not yet asserted itsrequest line. As a result, the EDMA state machine 250 transitions fromstate RBSY to state RDRQ to await assertion the signal S₋₋ DRQ. In thetransition, the signal D₋₋ XFER₋₋ EN is set low.

If the signal IDE₋₋ BUSY is deasserted, but the term (IDE₋₋ SD₋₋ REQ &|SLV₋₋ PRE) is not true, that indicates the request signal for the SDbus is not asserted or the slave preemption signal SLV₋₋ PRE isasserted. In response, the EDMA state machine 250 transitions from stateRBSY to state NGNT. In the transition, the signal D₋₋ IDE₋₋ SD₋₋ REQ isset low, and the block mask signal D₋₋ BLK₋₋ MASK is set high. In thetransition from state RBSY to state NGNT, the signals D₋₋ XFER₋₋ EN andD₋₋ IDE₋₋ WS are set low.

One PCICLK clock after we have entered into NGNT, the signal IDE₋₋ SD₋₋GNT is deasserted since the signal IDE₋₋ SD₋₋ REQ was set low one clockearlier. When the signal IDE₋₋ SD₋₋ GNT is deasserted, the EDMA statemachine 250 transitions from state NGNT to state GNT. In the transition,the signal D₋₋ IDE₋₋ SD₋₋ REQ is set high to request the SD bus.

Referring now to FIGS. 23A and 23B, logic used to generate variousrelevant signals is shown. The mask bits PRI₋₋ MASKED and SEC₋₋ MASKEDused to generate the signal DRQ₋₋ MASKED are provided by D-typeflip-flops 1508 and 1510, respectively. The signal PRI₋₋ MASKEDcorresponds to a control register bit for the primary IDE devices, whilethe signal SEC₋₋ MASKED corresponds to a control register bit for thesecondary IDE devices. Both flip-flops 1508 and 1510 are clocked byPCICLK and preset by the system reset signal RST₋₋. Thus, by default,the mask bits PRI₋₋ MASKED and SEC₋₋ MASKED are set high to mask theprimary and secondary channels of the EDMA controller 204. The D inputof the flip-flop 1508 is connected to the output of an OR gate 1512,whose inputs are connected to the outputs of AND gates 1514 and 1516.The three inputs of the AND gate 1514 receive signals D₋₋ DATA₋₋ VLD,PSID₋₋ TC, and PRI₋₋ SEC₋₋. Thus, upon completion of a data transfercycle, the mask bit PRI₋₋ MASKED is set high.

The first input of the AND gate 1516 is connected to the output of a2-to-1 multiplexor 1520 and the second input is connected to the outputof a 3-input AND gate 1522. The 0 input of the multiplexor 1520 isconnected to the signal PRI₋₋ MASKED, and the 1 input receives a latchedversion of PCI data bit 8 or PCI₋₋ WRDT 8!. The select input of themultiplexor 1520 receives an inverted byte enable signal |PCI₋₋ BE₋₋ 1!.The inputs of the AND gate 1522 receive the inverted state of a signalINDEX₋₋ DATA₋₋ which when set high indicates a write to an indexregister in the PCI-ISA bridge 130; a signal WR₋₋ IO indicating an I/Owrite; and a signal PID₋₋ CONTROL₋₋ EN to enable writing to the primarycontrol register. Thus, whether the primary IDE devices are maskeddepend on the state of bit PCI₋₋ WRDT 8! in an I/O write to the primaryIDE control register. The logic associated with the secondary mask bitSEC₋₋ MASKED is similar. The D input of the flip-flop 1510 is connectedto the output of an OR gate 1524, whose inputs are connected to theoutputs of AND gates 1526 and 1528. The three inputs of the AND gate1526 receive signals D₋₋ DATA₋₋ VLD, PSID₋₋ TC, and the inverted stateof the signal PRI₋₋ SEC₋₋. The first input of the AND gate 1528 isconnected to the output of an AND gate 1530, and the second input isconnected to the output of a multiplexor 1532. The inputs of the ANDgate 1530 receive the inverted state of the signal INDEX₋₋ DATA₋₋, thesignal WR₋₋ IO, and the signal SIP₋₋ CONTROL₋₋ EN which enables an I/Owrite to the secondary control register. The 0 input of the multiplexor1532 is connected to the signal SEC₋₋ MASKED, and the 1 input receivesthe latched PCI data bit PCI₋₋ WRDT 8!. The select input of themultiplexor 1532 receives the inverted byte enable signal |PCI₋₋ BE₋₋1!.

The primary and secondary IDE control registers each further contains abit indicating whether a memory write or memory read transfer isoccurring. To that end, signals PRI₋₋ EDMA₋₋ W₋₋ R₋₋ and SEC₋₋ EDMA₋₋W₋₋ R₋₋ are provided by D-type flip-flops 1534 and 1536, respectively.Both flip-flops are clocked by PCICLK and cleared by RST₋₋. An EDMAwrite to main memory 114 is indicated if either of signals PRI₋₋ EDMA₋₋W₋₋ R₋₋ or SEC₋₋ EDMA₋₋ W₋₋ R₋₋ is asserted high. The D input of theflip-flop 1534 is connected to the output of a two-input AND gate 1538,whose first input is connected to the output of the AND gate 1522 andwhose second input is connected to the output of a multiplexor 1540. The0 input of the multiplexor 1540 is connected to the signal PRI₋₋ EDMA₋₋W₋₋ R₋₋, and the 1 input receives the PCI latched data bit PCI₋₋ WRDT0!. The multiplexor 1540 is selected by the PCI byte enable signal|PCI₋₋ BE₋₋ 0!. Thus, an EDMA write or read is indicated by the state ofbit PCI₋₋ WRDT 0! during an I/O write to either the primary or secondarycontrol registers.

The D input of the flip-flop 1536 is provided by the output of an ANDgate 1542, whose first input is connected to the output of a multiplexor1544 and whose second input is connected to the output of the AND gate1530. The 0 input of the multiplexor 1544 is connected to the signalSEC₋₋ EDMA₋₋ W₋₋ R₋₋, and the 1 input receives the latched PCI data bitPCI₋₋ WRDT 0!. The multiplexor 1544 is selected by the byte enablesignal |PCI₋₋ BE 0!.

The mask bit PRI₋₋ MASKED is further provided to an inverted input of anAND gate 1546 (FIG. 23B), whose other input receives the primary IDErequest signal or IDE₋₋ DRQ₋₋ P. The output of the AND gate 1546 isconnected to the D input of a D-type flip-flop 1548, which is clocked byPCICLK. The output of the flip-flop 1548 provides the signal S₋₋ IDE₋₋DRQ₋₋ P. Similarly, the mask bit SEC₋₋ MASKED is provided to an invertedinput of a NAND gate 1547, whose other input receives the secondary IDErequest signal IDE₋₋ DRQ₋₋ S. The output of the AND gate 1547 isconnected to the D input of a D-type flip-flop 1550, which is clocked byPCICLK and whose output provides the signal S₋₋ IDE₋₋ DRQ₋₋ S. Thus, theprimary and secondary request lines S₋₋ IDE₋₋ DRQ₋₋ P and S₋₋ IDE₋₋DRQ₋₋ S provided to the EDMA state machine 250 contain the maskinginformation.

The write and read strobes IDE₋₋ WR₋₋ and IDE₋₋ RD₋₋ provided to the IDEdevices are driven by 3-to-1 multiplexors 1552 and 1554, respectively.The 0 input of the multiplexor 1552 is tied high, the one input receivesa signal IOWC₋₋, and the 2 input is connected to the output of an ORgate 1556. The two inputs of the OR gate 1556 receive an I/O strobesignal IO₋₋ and write/read signal IDE₋₋ LW₋₋ R₋₋. The signal IO₋₋ isprovided by the IDE state machine 252 and is provided to enableassertion of the write and read strobes. The S1 and S0 inputs of themultiplexor 1552 receive signals IDE₋₋ SD₋₋ GNT and IDE₋₋ DECODE,respectively. As noted above, when the CPU/main memory system 101requests a transfer to the IDE devices, it generates a read or writesector I/O command on the PCI bus P which is transmitted via the ISA busI to an IDE device. The I/O command includes an I/O addresscorresponding to one of the command registers in the selected IDEchannel, which then causes a signal IDE₋₋ DECODE in the EDMA controller204 to be asserted. When that occurs, the state of an internallygenerated signal O₋₋ IOWC₋₋ which mirrors the IOWC₋₋ signal of the ISAbus I determines the state of the write strobe IDE₋₋ WR₋₋. This initialassertion of the IDE₋₋ WR₋₋ strobe writes the necessary command bytesinto the selected IDE device. In the EDMA data transfer mode, the signalIDE₋₋ SD₋₋ GNT is asserted, and the state of the write/read signal IDE₋₋LW₋₋ R determines if the command strobe IDE₋₋ WR₋₋ is asserted.

Similarly, the 0 input of the multiplexor 1554 is tied high, the 1 inputreceives an internally generated read command signal O₋₋ IORC₋₋, and the2 input is connected to the output of an OR gate 1558. The inputs of theOR gate 1558 receive the signal IO₋₋ and the inverted state of thesignal IDE₋₋ LW₋₋ R₋₋. The S1 and S0 inputs of the multiplexor 1554 alsoreceive signals IDE₋₋ SD₋₋ GNT and IDE₋₋ DECODE.

Once the selected IDE device is ready to perform the data transfer, itresponds by asserting request lines IDE₋₋ DRQ₋₋ P or IDE₋₋ DRQ₋₋ S,which prompts the EDMA controller 204 to request the SD bus. When theIDE SD request is granted, the EDMA controller 204 provides acknowledgesignals IDE₋₋ DAK₋₋ P₋₋ or IDE₋₋ DAK₋₋ S₋₋ back to the IDE devices. Theacknowledge signals are provided by OR gates 1560 and 1562,respectively. The inputs of the OR gate 1560 receive a signal DAK₋₋ andthe inverted state of the synchronized primary/secondary signal S₋₋PRI₋₋ SEC₋₋. The inputs of the OR gate 1562 receive the signals DAK₋₋and S₋₋ PRI₋₋ SEC₋₋. The signal DAK₋₋ is provided by the IDE statemachine 252 to indicate when data transfer is enabled as indicated bythe signal XFER₋₋ EN.

The hit/miss signal HIT₋₋ MISS₋₋ is provided by the OR gate 1564, whoseinputs receive signals IDE₋₋ WRITE₋₋ HIT and IDE₋₋ READ₋₋ HIT providedby the outputs of AND gates 1566 and 1568, respectively. The first inputof the AND gate 1566 is connected to the output of a NAND gate 1570,which receives the write buffer empty signal BUF₋₋ EMPTY and latchedbyte enable signals LTBE₋₋ 1! and LTBE₋₋ 0! to indicate an odd wordaddress. The output of the NAND gate 1570 if high indicates that thewrite buffer is not empty. The remaining inputs of the AND gate 1566receive signals IDE₋₋ SD₋₋ GNT, IDE₋₋ LW₋₋ R, and SAME₋₋ PAGE. Thus, awrite hit occurs only when an odd word address is not indicated, the IDEgrant signal is asserted, a write operation is in progress, and the EDMAwrite is to the same page. The inputs of the AND gate 1568 receive thesignal IDE₋₋ SD₋₋ GNT, the inverted state of the signal IDE₋₋ LW₋₋ R₋₋,the signal SAME₋₋ PAGE, and the signal I₋₋ RD₋₋ VLD. Thus, a read hitoccurs if the SD grant signal is asserted, a read operation is inprogress, the EDMA read address is to the same page as the previousaddress, and the read data bits are valid.

Referring now to FIG. 24, the state diagram of the IDE state machine 252is shown. The IDE state machine 252 provides the following outputsignals: IDE I/O strobe or D₋₋ IO₋₋ signal; an acknowledge signal or D₋₋DAK₋₋ signal; a signal D₋₋ ACT₋₋ INACT₋₋ indicating the active orinactive period of an EDMA data transfer cycle; an address strobe D₋₋AD₋₋ STB signal; a signal D₋₋ LOAD₋₋ TIME for loading the active orinactive time into the countdown timer 1631 (FIG. 21); a signal D₋₋DATA₋₋ VLD asserted during the last cycle of the active period toindicate that data will be valid on the next rising PCICLK edge; asignal D₋₋ NEXT₋₋ RD indicating decode for the next read cycle; a signalEDMA₋₋ INC for incrementing and decrementing the address and byte countregisters keeping track of data transfer from the read and writebuffers; and a signal D₋₋ IDE₋₋ IDLE indicating that the IDE statemachine 252 is in the IDLE state. All the signals having "D₋₋ " prefixesprovide the D inputs of a series of D flip-flops clocked by PCICLK andcleared by RST₋₋ which provide corresponding output signals having theexact same nomenclature except without the "D₋₋ " prefix.

The following signals maintain their state unless otherwise noted: D₋₋IO₋₋, D₋₋ DAK₋₋, D₋₋ ACT₋₋ INACT₋₋, and D₋₋ LOAD₋₋ TIME. The followingsignals are set low unless otherwise noted: D₋₋ DATA₋₋ VLD, D₋₋ AD₋₋STB, D₋₋ NEXT₋₋ RD, EDMA₋₋ INC, and D₋₋ IDE₋₋ IDLE.

On system reset, indicated by the signal |RST₋₋, the IDE state machine252 enters state IDLE. In state IDLE, the signals D₋₋ IO₋₋, D₋₋ ACT₋₋INACT₋₋, D₋₋ LOAD₋₋ TIME, and D₋₋ IDE₋₋ IDLE are all set high. The statemachine remains in state IDLE until the transfer enable or XFER₋₋ ENsignal is asserted by the EDMA state machine 250. If the signal XFER₋₋EN is detected asserted, the state machine transitions from state IDLEto state DAK. In the transition, the acknowledge or D₋₋ DAK₋₋ signal isset low and the address strobe or D₋₋ AD₋₋ STB signal is set high. Asnoted, assertion of the signal DAK₋₋ causes either the IDE₋₋ DAK₋₋ P₋₋or IDE₋₋ DAK₋₋ S₋₋ signal to be asserted, depending on the state of theprimary/secondary signal PRI₋₋ SEC₋₋. Assertion of the signal D₋₋ AD₋₋STB strobes in the EDMA address to determine if a hit or miss hasoccurred in the read or write data buffers.

In state DAK, if the signal XFER₋₋ EN is deasserted by the EDMA statemachine 250, then the IDE state machine 252 returns to state IDLE. Inthe transition from state DAK to state IDLE, the signal D₋₋ DAK₋₋ is sethigh. If the signal XFER₋₋ EN is asserted but the signal IDE₋₋ WS isalso asserted, then the state machine remains in state DAK. The IDEstate machine 252 waits until the wait state signal IDE WS is deassertedby the EDMA state machine 250.

If the signal IDE₋₋ WS is deasserted and the signal XFER₋₋ EN isasserted, then the IDE state machine 252 transitions from state DAK tostate ACT. In the transition, the signal D₋₋ LOAD₋₋ TIME is set high. Asdiscussed with respect to FIG. 21, asserting the signal LOAD₋₋ TIMEcauses the active/inactive timer 1631 (FIG. 21) to be loaded with theappropriate value from either the primary or secondary active timingregister 1600 or 1602. Further, the signal D₋₋ IO₋₋ is set low to enablethe IDE I/O write and read commands IDE₋₋ WR₋₋ and IDE₋₋ RD₋₋ ; thesignal D₋₋ ACT₋₋ INACT₋₋ is set high to indicate the active period; thesignal D₋₋ NEXT₋₋ RD is set high to latch in the next read address toselect (RD₋₋ SEL 1:0!) the appropriate double word of the read databuffer 211; and the signal EDMA₋₋ INC is set high to increment ordecrement the address and byte counters.

The IDE state machine 252 remains in state ACT while the signal TIME₋₋CNT₋₋ 0 is deasserted. In state ACT, the signal D₋₋ LOAD₋₋ TIME is setlow. When the active/inactive timer 1631 counts down to zero and assertsthe signal TIME₋₋ CNT₋₋ 0 to indicate the end of the active period, thestate machine transitions from state ACT to INACT. In the transition,the signal D₋₋ LOAD₋₋ TIME is set high to load in the inactive timeperiod into the timer 1631 from either the primary or secondary inactivetiming register 1604 or 1606; the signal D₋₋ AD₋₋ STB is set high tostrobe in the EDMA address; the signal D₋₋ IO₋₋ is set high to disablethe IDE I/O command strobe IDE₋₋ WR₋₋ or IDE₋₋ RD₋₋ ; the signal D₋₋ACT₋₋ INACT is set low to indicate the inactive period; and a signal D₋₋DATA₋₋ VLD is set high to indicate that data has been transferred.

In state INACT, the signal D₋₋ LOAD₋₋ TIME is set low, and if the signalXFER₋₋ EN is deasserted, the signal D₋₋ DAK₋₋ is set high to deactivethe IDE acknowledge signals IDE₋₋ DAK₋₋ P₋₋ or IDE₋₋ DAK S₋₋. There arethree possible transitions out of state INACT. If the signal TIME₋₋CNT₋₋ 0 is asserted indicating that the inactive period has elapsed, thesignal XFER₋₋ EN is asserted, the signal |DAK₋₋ is true indicating thatthe signal XFER₋₋ EN has not been deasserted by the EDMA state machine250, and the signal IDE₋₋ WS is asserted indicating a wait state, theIDE state machine 252 transitions from state INACT to state DAK. If thesignal TIME₋₋ CNT₋₋ 0 and XFER₋₋ EN are asserted, but the signal DAK₋₋is deasserted, then that indicates that XFER₋₋ EN signal must have beendeasserted by the EDMA state machine 250 and reasserted again. In thiscase, the state machine also transitions from state INACT to state DAK,setting the signal D₋₋ DAK₋₋ low to re-enable the acknowledge signalsprovided to the IDE devices.

If, however, the signals TIME₋₋ CNT₋₋ 0, XFER₋₋ EN, and DAK₋₋ areasserted, but the signal IDE₋₋ WS is deasserted, then that indicatesthat the next data transfer cycle can continue. As a result, the statemachine transitions from state INACT to state ACT. In the transitionfrom state INACT to state ACT, the signal D₋₋ LOAD₋₋ TIME is set high toload the active/inactive timer 1631 with the active time period; thesignal D₋₋ IO₋₋ is set low to enable the IDE I/O command strobe IDE₋₋WR₋₋ or IDE₋₋ RD₋₋ ; the signal D₋₋ ACT₋₋ INACT₋₋ is set high toindicate the active period; the signal EDMA₋₋ INC is set high to enablethe increment and decrement of the address and byte counters; and thesignal D₋₋ NEXT₋₋ RD is set high to latch in the next read address forselecting the appropriate double word.

Finally, if the signal TIME₋₋ CNT₋₋ 0 is asserted but the signal XFER₋₋EN is deasserted, the IDE state machine 252 returns from state INACT tostate IDLE.

Referring now to FIG. 25, a state diagram of the write latch enablestate machine 254 is shown. As noted above, the state machine 254provides the write latch enable signals IDE₋₋ WR₋₋ LE 7:0! for latchingdata into the first stage of the write buffers comprising latches1302A-H in FIG. 19. In state WR0, the signals IDE₋₋ WR₋₋ LE 7:0! are setequal to the value 0x03. This enables latches 1302A-B. The state machine254 also provides signals IDE₋₋ WR₋₋ BE 7:0! which are multiplexed ontothe byte enable bits C/BE₋₋ 3:0! on the PCI bus P. In state WR0, thebyte enable signals IDE₋₋ WR₋₋ BE 7:0! are assigned the value 0x00.

If the signal DATA₋₋ VLD is asserted indicating a valid data transfer,the signal IDE₋₋ LW₋₋ R₋₋ is set high indicating a write, and the signalHIT₋₋ MISS₋₋ is high indicating a hit to the data write buffers, thenthe state machine 254 transitions to state WR1. In the transition, thewrite latch enable signals IDE₋₋ WR₋₋ LE 7:0! are assigned the value0x02 and the byte enable signal IDE₋₋ WR₋₋ BE 0! is set high, indicatinga valid word of data has been latched. If a miss occurs, indicated bythe signal HIT₋₋ MISS₋₋ being set low and the signals DATA₋₋ VLD andIDE₋₋ LW₋₋ R₋₋ being high, then the state machine transitions from stateWR0 to state LTCH, where the write latch enable signals IDE₋₋ WR₋₋ LE7:0! are assigned a value 0x00 and the byte enable bit IDE₋₋ WR₋₋ BE 1!is set high, indicating this was odd word aligned as this is the onlytime a miss will occur on the first transfer.

The state machine remains in state LTCH until the PCI master logic 206asserts the address/data register clock WRADR. As explained above inrelation to the EDMA state machine 250, a miss indicated by the signalHIT₋₋ MISS₋₋ causes the EDMA state machine 250 to assert the PCI masterrequest signal IDE₋₋ REQ, which in turn causes the PCI master logic 206to post the data in the second level write buffers and run the cycle onthe PCI bus P. The PCI master logic 206 asserts the signal WRADR when ittransitions out of its IDLE state to latch in the PCI address on thenext PCI cycle. When that occurs, the state machine 254 transitions fromstate LTCH to state WR0 with the signals IDE₋₋ WR₋₋ LE 7:0! beingassigned the value 0x03 and the byte enable signals IDE₋₋ WR₋₋ BE 7:0!being assigned the value 0x00.

Next, in state WR1, the state machine transitions to state WR2 if thesignal DATA₋₋ VLD is asserted and the signal HIT₋₋ MISS₋₋ is assertedhigh. In this transition, the write latch enable signals IDE₋₋ WR₋₋ LEare set equal to the value 0x04 while the byte enable signal IDE₋₋ WR₋₋BE 1! is set high. The state machine transitions from state WR1 to stateLTCH if the signal DATA₋₋ VLD is high and the signal HIT₋₋ MISS₋₋ is lowindicating a miss. In the transition, the signal IDE₋₋ WR₋₋ BE 1! isasserted. The state machine also transitions to state LTCH if the signalDRQ₋₋ PRE is asserted high indicating a bus master preemption.Otherwise, if the signal DATA₋₋ VLD is deasserted, the state machineremains in state WR1.

The same conditions causing transitions out of state WR1 apply to stateWR2. The state machine remains in state WR2 if the signal DATA₋₋ VLD isdeasserted low. The state machine transitions from state WR2 to stateWR3 if a data transfer is indicated by the signal DATA₋₋ VLD and thesignal HIT₋₋ MISS₋₋ is asserted high. In the transition to state WR3,the write latch enable signals IDE₋₋ WR₋₋ LE 7:0! are assigned the value0x08, while the byte enable signal IDE₋₋ WR₋₋ BE 3! is set high. If thesignal DATA₋₋ VLD is asserted and a miss is indicated by the signalHIT₋₋ MISS₋₋, the state machine transitions to state LTCH, setting thesignal IDE₋₋ WR₋₋ BE 3! high. Additionally, if a bus master preemptionoccurs and S₋₋ DRQ is not asserted, which is indicated by the signalDRQ₋₋ PRE, the state machine also transitions to state LTCH.

The conditions for transitioning out of state WR3, WR4, WR5 and WR6 arethe same as those for WR1 or WR2, except that the write latch enablesignals IDE₋₋ WR₋₋ LE are assigned the value 0x10 in the transition fromstate WR3 to state WR4, the value 0x20 in the transition from state WR4to state WR5, the value 0x40 in the transition from state WR5 to stateWR6, and the value 0x80 in the transition from state WR6 to state WR7.Further, the signal IDE₋₋ WR₋₋ BE 3! is set high in the transition outof state WR3 with the signal DATA₋₋ VLD asserted, the signal IDE₋₋ WR₋₋BE 4! is set high in a transition out of state WR4 with the signalDATA₋₋ VLD asserted, the signal IDE₋₋ WR₋₋ BE 5! is set high in atransition out of state WR5 with the signal DATA₋₋ VLD asserted, and thesignal IDE₋₋ WR₋₋ BE 6! is set high in a transition out of state WR6with the signal DATA₋₋ VLD set high.

The state machine remains in state WR7 until the final data transfer hasoccurred as indicated by the signal DATA₋₋ VLD. On the assertion of thesignal DATA₋₋ VLD, the state machine transitions from state WR7 to stateLTCH, setting the signal IDE₋₋ WR₋₋ BE 7! high. The state machine alsotransitions from state WR7 to state LTCH if a bus preemption occurs andS₋₋ DRQ is not asserted, which is indicated by the signal DRQ₋₋ PRE.

Thus, an improved DMA controller has been described having programmabledata transfer timings. Not only is the total cycle time programmable,but the active and inactive period of the cycle are also programmable.An active timing register and an inactive timing register are used inconjunction with a countdown timer to determine the active and inactiveperiods of the data transfer cycle. The active time period is loadedinto the timer during the active phase, with the end of the active phasebeing indicated by the timer timing out. Next, the inactive time periodis loaded into the timer, which similarly times out to indicate the endof the inactive phase of the data transfer cycle.

While the preferred embodiment uses identical timings for the master andslave devices on the primary and secondary IDE buses, individual timingsfor master and slave devices could be provided if desired by providingadditional timing registers and multiplexers to the timing circuitry ofFIG. 21.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the-illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

We claim:
 1. A computer system, comprising:a microprocessor; a memorydevice; a CD-ROM drive; a bus on which write cycles and read cycles arerun; said memory device and said CD-ROM being coupled to said bus; acontroller for controlling data transfers between said memory device andsaid CD-ROM drive, each data transfer being performed in one datatransfer cycle defined by an active period and an inactive period, saidcontroller comprising:a first storage device for storing a valuerepresenting the active period, said active period value capable ofbeing any one of a number of different values; a second storage devisefor storing a value representing the inactive period, said inactiveperiod value capable of being any one of a number of different values; adata transfer circuit coupled to said bus and transferring data fromsaid CD-ROM drive to the memory device in a data transfer cycle of awrite cycle and from said memory device to said CD-ROM drive in a datatransfer cycle if a read cycle, said transfer of data occurring duringthe active period of the data transfer cycle; and a timer coupled tosaid first and second storage devices and to said data transfer circuitfor indicating when the end of the active period occurs and when the endof the inactive period occurs based on said active period and inactiveperiod values stored in said first and second storage devices.
 2. Thecomputer system of claim 1, further comprising:a signal provider coupledto said timer and to said data transfer circuit and providing anactive/inactive signal, said active/inactive signal being at a firststate if the active period is in progress, and said active/inactivesignal being at a second state if the inactive period is in progress;and a multiplexor having a select input receiving said active/inactivesignal, a first input connected to said first storage device, a secondinput connected to said second storage device, and an output connectedto said timer, wherein said active period value is loaded into saidtimer if said active/inactive signal is at said first state, and whereinsaid inactive period value is loaded into said timer if saidactive/inactive signal is at said second state.
 3. The computer systemof claim 1, wherein said first storage device is a first register havinga plurality of bits, and wherein said second storage device is a secondregister having a plurality of bits.
 4. The computer system of claim 1,wherein first and second storage devices are loaded with said activeperiod and inactive period values during a configuration write cycle onsaid bus.
 5. The computer system of claim 4, wherein said bus isaccording to the peripheral connect interface (PCI) standard, andwherein said configuration write cycle is a PCI configuration writecycle.
 6. The computer system of claim 1, wherein said CD-ROM is anintegrated drive electronics (IDE) device.
 7. The computer system ofclaim 1, wherein said CD-ROM drive comprises a plurality of CD-ROMdrives, and wherein said data transfer circuit transfers data betweenany of said plurality of said CD-ROM drives and said memory device, saidcontroller further comprising:an indicator coupled to said datatransferring means and indicating which of said plurality of CD-ROMdrive is selected.
 8. The computer system of claim 1 wherein each ofsaid plurality of CD-ROM drives provides a request signal to indicatewhen asserted that the corresponding CD-ROM drive is requesting atransfer of data between said CD-ROM drive and said memory device, andwherein said indicator means receives said plurality of request signalsto determine which of said plurality of I/O devices is selected.
 9. Thecomputer system of claim 1, further comprising:a plurality of firststorage devices corresponding to said plurality of CD-ROM drives eachfor storing a value representing the active period for a correspondingCD-ROM drive wherein said active period value stored in each of saidplurality of first storage devices is capable of being any one of adifferent number of values; and a plurality of second storage devicescorresponding to said plurality of CD-ROM drives each for storing a clueindicating the inactive period for a corresponding CD-ROM drive whereinsaid inactive period values stored in each of said plurality of secondstorage devices is capable of being any one of a different number ofvalues; and a multiplexor having inputs coupled to said plurality offirst and second storage devices and an output coupled to said timer,wherein said multiplexor loads a value from one of said plurality offirst and second storage devices depending on the CD-ROM drive selectedand if the data transfer cycle is in the active or inactive period. 10.The indicator system of claim 9, wherein each of said plurality ofCD-ROM drives is an integrated drive electronics (IDE) device.
 11. Thecomputer system of claim 1, further comprising:a monitor.
 12. A computersystem, comprising:a microprocessor; a memory device; a hard disk drive;a bus on which write cycles and read cycles are run; said memory deviceand said hard disk being coupled to said bus; a controller forcontrolling data transfers between said memory device and said hard diskdrive, each data transfer being performed in one data transfer cycledefined by an active period and an inactive period, said controllercomprising:a first storage device for storing a value representing theactive period, said active period value capable of being any one of anumber of different values; a second storage device for storing a valuerepresenting the inactive period, said inactive period value capable ofbeing any one of a number of different values; a data transfer circuitcoupled to said bus and transferring data from said hard disk drive tothe memory device in a data transfer cycle of a write cycle and fromsaid memory device to said hard disk drive in a data transfer cycle if aread cycle, said transfer of data occurring during the active period ofthe data transfer cycle; and a timer coupled to said first and secondstorage devices and to said data transfer circuit for indicating whenthe end of the active period occurs and when the end of the inactiveperiod occurs based on said active period and inactive period valuesstored in said first and second storage devices.
 13. The computer systemof claim 12, further comprising:a signal provider coupled to said timerand to said data transfer circuit and providing an active/inactivesignal, said active/inactive signal being at a first state if the activeperiod is in progress, and said active/inactive signal being at a secondstate if the inactive period is in progress; and a multiplexor having aselect input receiving said active/inactive signal, a first inputconnected to said first storage device, a second input connected to saidsecond storage device, and an output connected to said timer, whereinsaid active period value is loaded into said timer if saidactive/inactive signal is at said first state, and wherein said inactiveperiod value is loaded into said timer if said active/inactive signal isat said second state.
 14. The computer system of claim 12, wherein saidfirst storage device is a first register having a plurality of bits, andwherein said second storage device is a second register having aplurality of bits.
 15. The computer system of claim 12, wherein firstand second storage devices are loaded with said active period andinactive period values during a configuration write cycle on said bus.16. The computer system of claim 15, wherein said bus is according tothe peripheral connect interface (PCI) standard, and wherein saidconfiguration write cycle is a PCI configuration write cycle.
 17. Thecomputer system of claim 12, wherein said hard disk drive is anintegrated drive electronics (IDE) device.
 18. The computer system ofclaim 12, wherein said hard disk drive comprises a plurality of harddisk drives, and wherein said data transfer circuit transfers databetween any of said plurality of said I/O devices and said memorydevice, said controller further comprising:an indicator coupled to saiddata transferring means and indicating which of said plurality of harddisk drive is selected.
 19. The computer system of claim 12 wherein eachof said plurality of hard disk drives provides a request signal toindicate when asserted that the corresponding hard disk drive isrequesting a transfer of data between said hard disk drive and saidmemory device, and wherein said indicator means receives said pluralityof request signals to determine which of said plurality of I/O devicesis selected.
 20. The computer system of claim 12, further comprising:aplurality of first storage devices corresponding to said plurality ofhard disk drives each for storing a value representing the active periodfor a corresponding hard disk drive wherein said active period valuestored in each of said plurality of first storage devices is capable ofbeing any one of a different number of values; and a plurality of secondstorage devices corresponding to said plurality of hard disk drives eachfor storing a clue indicating the inactive period for a correspondinghard disk drive wherein said inactive period values stored in each ofsaid plurality of second storage devices is capable of being any one ofa different number of values; and a multiplexor having inputs coupled tosaid plurality of first and second storage devices and an output coupledto said timer, wherein said multiplexor loads a value from one of saidplurality of first and second storage devices depending on the hard diskdrive selected and if the data transfer cycle is in the active orinactive period.
 21. The indicator system of claim 20, wherein each ofsaid plurality of hard disk drives is an integrated drive electronics(IDE) device.
 22. The computer system of claim 12, further comprising:amonitor.